* relative to Black, Watt, and Cavendish. 487 



This supposition, that the most permanent airs are of a me- 

 tallic origin and nature, representing hydrogen for instance 

 as ferreous gas, was set aside by the experiments of Cavendish, 

 which proved that the gas from iron is identical with the gas 

 from zinc, in specific gravity, in explosive power, in the quan- 

 tity in which it combines with oxygen, and in the result of the 

 combination : they went also, as far as our experiments reach, 

 to invalidate the general supposition that the repulsive force 

 of the particles of matter is in proportion to their weight and 

 density; since they proved that hydrogen is, both in its elastic 

 and in its fixed state, the lightest of bodies; unless indeed its 

 high refractive power should be thought a stronger argument 

 for the density, than its low combining weight for the light- 

 ness, of its molecules. 



Newton seems not to have been aware, that the facts of the 

 condensation of one gas and permanence of another, the ob- 

 servation of which he here ascribes to Huygens, had been 

 established some ten years before by experiments instituted at 

 the Royal Society, in which his correspondent Boyle had as- 

 sisted — a circumstance however which was notified to the 

 public when Huygens's paper was printed*. The experiments 

 themselves having, I think, never been published, the interest 

 which we equally take in tracing back the history of science, 

 the curiosity of the experiments, and the celebrity of the ex- 

 perimenters, prompt me to give you some extracts on this 

 subject from the journals of the Society. 



From these it appears that on January the 4th, 1664, a 

 year before the publication of the Micrographia, Hook exhi- 

 bited to the Society " experiments to show that air is the uni- 

 versal dissolvent of sulphureous [combustible] bodies, and that 



* An account of Huygens's experiments was printed at Paris in 1674, 

 and appears in the Philosophical Transactions, No. 119, dated November 

 22, 1675, under the title of — " some experiments made in the air-pump by 

 M. Papin directed by M. Hugens." The following extract contains the 

 facts to which Newton referred: — " The experimenter being desirous to see 

 whether these ebullitions did make new air, put in the recipient a gage, 

 and observed that when the liquors were mingled, the water in the gage 

 rose very nimbly to the top of the gage; and drawing out the new air he 

 made the gage-water subside again; and by this means it was seen, that all 

 these kinds of ebullition make an air which expands itself like common air. 

 Yet here is something that seems to be very remarkable, which is, that the 

 air made by these ebullitions is not of the same nature : for it has been found 

 experimentally, that the air formed by the mixture of aquafortis and copper 

 remains always air, and always keeps up the water in the glass; but on the 

 contrary, the air which has been made by the mixture of oil of tartar and 

 oil of vitriol is almost all destroyed of itself, in the space of twenty-four 

 hours. All these ebullitions hitherto spoken of are greater in vacuo than 

 in the open air; but with lime it is not so." 



