which both depend is a tafte for fuperfluity in those who have an equivalent to give; this tafte is what produces demand, and this again is the main spring of the whole operation.

CHAP. II. We have substituted throughout this book, the term demand, to express the idea we conveyed in the last by that of wants; and fince the subject becomes more complex, and that we have many more relations to take in, I must make a recapitulation of all the different acceptations of this term demand.

Demand, in the first place, is always relative to merchandize ; it is the buyer who demands; the feller offers to fale. 2. It is faid to be reciprocal, when there is a double operation, that is, when the feller in the first, becomes the buyer in the second cafe; and then, taking the two operations in one view, we call those demanders who have paid the higheft price. 3. Demand is fimple or compound; fimple, when there is no competition among the buyers; compound, when there is. 4. It is great or small, according to the quantity demanded. And 5. High or low, according to the price offered. The nature of a gradual encrease of demand, is to encourage induftry by augmenting the supply; that of a Judden encreafe, is to make prices rife. This principle has not every where the fame efficacy in producing these varieties it is checked in its operations between merchants, who feek their profit; and it is accelerated among private people, who feek for fubfiftence, neceffaries, or luxurious gratifications.

:

CHAP. III. I come next to deduce the origin of trade and induftry, which I difcover from the principles of the first book, where bartering of neceffaries was understood to be trade; and I find that the progress of this is owing to the progress of multiplication and agriculture. When a people arrive at a moral impoffibility of increasing in numbers, there is a ftop put to the progrefs of barter. This grows into trade, by the introduction of a new want (money) which is the universal object of defire to all men. While the defires of man are regulated by their phyfical wants, they are circumfcribed within certain limits. So foon as they form to themselves others of a political nature, then all bounds are broken down. The difficulty of adapting wants to wants, naturally introduces money, which is an adequate equivalent for every thing. This conftitutes fale, which is a refinement on barter. Trade is only a step farther; it is a double fale, the merchant buys, not for himself, but for others. A merchant is a machine of a complex nature. Do you want, he fupplies you; have you any fuperfluities, he relieves you of them; do you want fome of the univerfal equivalent money, he gives it you, by creating in you a credit in proportion to your circumftances. The introduction of fo useful a machine,

prompts

prompts every one to wish for the power of using it; and this is the reason why mankind extend their labour beyond the mere supply of their phyfical wants.

Trade therefore abridges the tedious operations of fale and barter, and brings to light many things highly important for individuals, who live by relieving the wants of others, to know. It makes the standard of demand, which is, in a manner, the voice of the statesman, conducting the operations of industry towards the relief of wants; and directing the circulation of fubfiftence towards the habitations of the neceffitous.'

We have given this fpecimen of our author's manner as a proof how well he understands his fubject; and to shew, that the science of government is a study far from being fo fimple, or so easily attained, as is commonly imagined.

[To be continued. ]

II. The Hiftory of the prefent State of Electricity, with Original Experiments, by Jofeph Priestley, L. L. D. F. R. S. 4to. Price 11. 15. Cadell.

NR

O branch of natural philofophy hath been fo confiderably extended and improved by the philofophers of the prefent age, as that which is called electricity. Thefe improvements and difcoveries have been gradual; they were made at different times, by different people, and in different countries, and confequently are only to be learnt from a great variety of books in various languages. It muft, therefore, neceffarily be very agreeable to those who are engaged in electrical enquiries, to fee the whole collected into one point of view, by a candid and judicious writer, who is perfectly acquainted with the subject.

Dr. Priestley divides his work into eight parts; the first of which contains the hiftory of the difcoveries of all the celebrated electricians, in the order of time in which they were published to the world. This hiftory he divides into ten periods. The firft of these comprehends the difcoveries prior to thofe of Mr. Hawkesbee. In the fecond we are presented with thofe of Mr. Hawkefbee himself. The third period contains the discoveries of Mr. Stephen Grey, prior to thofe of Mr. Du Faye, which bring the history of electricity to the year 1733. In the fourth period we find the discoveries of Mr. Du Faye. Period the fifth continues and concludes Mr. Grey's experiments. Those of Dr. Defaguliers conftitute the fixth. In period the feventh we are prefented with the experiments of the Germans, together with those of Dr. Watson, previous to the

discovery

discovery of the Leyden phial in the year 1746. Period the eighth contains the hiftory of electricity from the Leyden phial, to the discoveries of Dr. Franklin. The ninth displays the difcoveries and experiments of the last mentioned incomparable electrician; and in the tenth, the history is brought down to the year 1766. It is fufficient to obferve, relative to this firft part of the work, that it contains a very accurate and faithful hiftory of the birth and progress of this youngest fifter of the sciences, related in fuch a manner as to be at once both entertaining and instructive.

The fecond part comprehends a series of propofitions, in which all the general properties of electricity are comprised. In part the third we find the different theories of various authors who have written on this fubject, particularly thofe of pofitive and negative electricity, and of two electric fluids. Part the fourth contains Defiderata in electricity, and hints for the further extenfion of it. These hints, for the promotion

of further difcoveries, we fhall transcribe.

I. Concerning the Electrical fluid.

What is the proportion of the feveral colours in electric light, in different cafes, and in different appearances of it?

Is not the electric light a real vapour ignited, fimilar to that of phosphorus ; and may not experiments be, hereafter, made, where we fhall have the explofion, the shock, and the other effects of electricity, without the light? Is the electric light ever vifible except in vacuo? In the open air the electric fluid makes itself a vacuum in order to its paffage.

Collect the electric fluid, not from the general mass of the earth but from bodies of particular kinds, and observe if it have any different properties, with respect to light, &c.

Is it exactly the fame at fea, as on land; below the furface of the earth as above it, &c. &c. &c. ?

• Dr. Franklin obferved, that iron was corroded by being expofed to repeated electric fparks. Muft not this have been effected by fome acid? What other marks are there of an acid in the electric matter? May not its phosphoreal smell be reckoned one? Is it not poflible to change blue vegetable juices into red by fome application of electricity? This, I think, I have been told has been done at Edinburgh.

Is there only one electric fluid, or are there two? Or is there any electric fluid fui generis, at all, diftinct from the ether of Sir Ifaac Newton? If there be, in what refpect does it differ from the ether?

• Are the particles which affect the organ of fmelling, as well as the particles of light, parts of the proper electric fluid,

or

or are they merely adventitious, being, fome way or other, brought into action by electricity?

• Does not fome particular order of the particles, which Sir Ifaac Newton fuppofes to be continually flying from the furfaces of all bodies, conftitute the electric fluid; as others, he imagined, conftituted the air, and others the ether, &c.?.

Is it probable that there is even any temporary, or growing addition to, or dimunition of the whole ftock of electricity?

Whence arifes the elafticity of the electric fluid, and according to what law do its particles repel one another? Mr. Price.

II. Concerning Electrics and Conductors.

In what does the difference between electrics and conductors confift? In other words, what is it that makes fome bo, dies permeable to the electric fluid, and others impermeable to it?

• Are the pores of electric bodies smaller than thofe of conductors, and do they contain very much, or very little of the electric fluid?

"What is it in the internal ftructure of bodies that makes them break with a polish? Perhaps all folid electrics do so.

Has elafticity any connection with electricity, fome electrics being extremely elastic?

What is the reason why, in fome of Mr. Hawkesbee's experiments, the electric light was vifible through a confiderable thickness of very opaque electrics, as rofin, fulphur, pitch, &c; but not through the thinneft metallic conductors?

• What fimilarity is there in the proceffes of calcination, vegetation, animalization, and in fome measure chryftalization; fince all bodies which have gone through any of those proceffes, and perhaps no others, are found to be electrics?

Are not both electrics and conductors more perfect in their kind in proportion to their specific gravity?

Will not water conduct electricity the beft in its state of greatest condenfation; and metals the leaft in their greatest expanfion, as shown by a pyrometer ?

Try the conducting power of different metals, by fending a large fhock through wires of the fame fize, and obferving the different lengths that may be melted of the different wires. Dr. Franklin.

Compare the invifible effluvia of water with the invisible effluvia of a burning candle, and alfo thofe proceeding from other bodies, with refpect to their power of conducting elec tricity.

Obferve

• Obferve what degree of heat will discharge any given degree of electricity, in order to find in what degree heat makes air a conductor.

III. Concerning Excitation.

What is the difference, in the internal ftructure of electrics, that makes fome of them excitable by friction, and others by heating and cooling?

What have friction, heating, cooling, and the feparation after close contact in common to them all? How do any of them contribute to excitation? And in what manner is one, * or the other kind of electricity produced by rubbers and electrics of different furfaces ?

Is not Epinus's experiment of preffing two flat pieces of glafs together, when one of them contracts a pofitive and the other a negative electricity, fimilar to the experiments of Mr. Wilke, concerning the production of electricity by the liquefaction of various substances in others; when the substance which melts and contracts is in one ftate, and that which contains it is in the oppofite? And are not both these cases fimilar to the excitation of the tourmalin, &c. by heating and cooling? In this cafe, may not the tourmalin and the air act upon one another and be in opposite states?

Is not the circumftance common to all these cafes, fome affection of that space near the furface of the bodies in which the refractive power lies? When bodies which have been preffed together within that space recede from one another, more furface, and confequently more of that space is made, doth not the electric fluid flow into it from that body which has the leaft power of retaining it, and which it can permeate with the moft ease; when not being able to enter the substance of the other it refts upon its furface?

• Are not the particles of the electric and rubber thrown into a vibration in the act of excitation, which makes frequent recedings of the parts from one another, and thereby promotes the effect above-mentioned ?

• What is the real effect of putting moisture or amalgam upon the rubber? Do not those substances increase the power of excitation, as conductors more diftant from the smooth glass, in the gradation of electrics, than the surface of the leather? Or do they only make the rubber touch in more points, or alter the furface of the rubber?

Has that difference of furface on which colour depends any influence upon the power of excitation?

The