534 Scientific Proceedings, Royal Dublin Society. 



reduce tlie pressure to one millionth of an atmosphere (which is- 

 not very far from the greatest exhaustion that can be attained) the 

 average spacing of the molecules will have increased up to being- 

 equal to the ordinate of our gauge at a distance of about a metre 

 from the apes. It is instructive to observe that even this enlarged 

 interval is ultra- visible, and that in this so-called extreme vacuum 

 there remain something like a million millions of molecules of the 

 gas in every cubic millimetre of the space within the receiver — i. e. 

 about a thousand in every cubic micron. 



3. The magnitude to be next considered is the diameter of a. 

 molecule. By this is to be understood the distance within which 

 the centres of two molecules must come, if they are sensibly to 

 deflect each other's path. This size of the gaseous molecule as it may 

 be called, is intimately related to the ratio of the last two measures 

 to one another, and may be deduced from that ratio. Or it may be 

 obtained by observing the condensation which a gas or vapour 

 undergoes when passing into the state of a liquid or solid. Esti- 

 mated in either of these ways, it appears to be usually about the 

 8th, 10th, or 12th of the last measure — that is, it is something like 

 the ordinate of our gauge at a distance of one millimetre from its 

 apex. 



VI. — Smaller Magnitudes. 



The diameter of a gaseous molecule, as above defined, is the 

 smallest measurement for which the present gauge is suggested as 

 convenient, as it is also the smallest magnitude of the actual size 

 of which any approximate estimate has been made. But we have 

 through the spectroscope, indications of important events in nature 

 that are perpetually going on ivithin each gaseous molecule, and 

 probably on a very much smaller scale. For example, an easy 



as a mass probably not more tban a few times more or a few times less than the twenty- 

 fifthet or twenty-sixthet of a gramme. This seems the best approach that can at present 

 be made to estimating the mass of a chemical atom. 



The determination depends upon the average spacing of the centres of the mole- 

 cules of a gas at standard temperature and pressure (see last footnote) ; and if this very 

 important physical magnitude, which is common to all perfect gases, can be ascertained 

 "with more accuracy, we shall get a proportionally better estimate of the mass of a 

 chemical atom. Of course if the mass of the atom of any one element, e. g. hydrogen, 

 be determined, the masses of all the others become known by the chemical tables of 

 atomic weights. 



