VOh. LXIX.] PHILOSOPHICAL TRANSACTIONS, 557 



by heat, communicated to it in tlie same moment of its extrication; and that 

 the force of inflammable explosive air can only be proportionable to the sudden 

 expansion by heat in the moment of the inflammation, for no new extrication 

 here takes place; yet I did not consider enough in the account the suddenness of 

 this expansion, which may make a considerable difference in the force of the ex- 

 plosion. And indeed the above-mentioned experiments seem to demonstrate, 

 that the inflammation of the compound of pure dephlogisticated and aether air 

 spreads with such a velocity through the whole mass as to be almost instan- 

 taneous. 



It is well known, that mechanical power chiefly depends on the velocity with 

 which a body is endowed in the instant of exerting it; or that the momentum, 

 or force of a body, must be computed by multiplying the quantity of matter 

 into the velocity with which it moves. Thus, if this new compound of dephlo- 

 gisticated and aether air expands with 10 times greater velocity than any other 

 inflammable explosive air, its force will be about 10 times greater. 



As it seems to be probable, from what is already said, that this compound of 

 explosive air may be put to more uses than that of an amusing experiment, I 

 think it worth while, for men engaged in this branch of natural philosophy, to 

 look out for a method of producing at pleasure any quantity of dephlogisticated 

 air required. Considering the rapid progress which is daily made on the impor- 

 tant subject of air, I cannot but flatter myself that this great discovery is not far 

 off". The benefit which would arise from such a discovery for animal life, must 

 encourage every philosopher to pursue this object. Indeed, if we consider that 

 nitre contains this wonderful aerial fluid in a most concentrated state, and that 

 the nitrous acid seems to be nothing else but this beneficial fluid combined with 

 phlogiston, which seems to be imbibed by the vegetable alkali, when the acid is 

 expelled by heat in the form of this air; that this beneficial aerial fluid exists also, 

 in a most concentrated state, in bodies almost every where to be found, as are 

 calces of metal, principally that of iron; that common water contains it in great 

 abundance, so that the light and warmth of the sun extracts it to -J^- of the bulk 

 of the water, as Dr. Priestley found, that even the mass of our atmosphere is 

 nothing else but this very air soiled with impurities. If we consider, I say, all 

 this, is it not reasonable to hope, that we are near the important instant, when 

 this salubrious aerial fluid will be procured for many useful purposes in a suflicient 

 quantity, either by the discovery of a ready way to let loose this air from the 

 bodies in which it is as it were imprisoned, or by filtrating or purifying common 

 air from its impurities? 



XXP'Il. The Description of Two Neiv Micrometers. By Mr. Ramsden, 



Optician, p. 419. 



When an observer finds, that with the micrometer, which depends on move 



