516 Mr. J. J. Waterston on Chemical Notation. 



cal notation be founded thereupon [upon the fact (1) only, not 

 upon the hypothesis (2) (see note), or upon any hypothesis], and 

 we obtain with perfect conciseness at once the chemical com- 

 position, the physical density of a unit volume, and the con- 

 stitution of a molecule." On this Mr. Odling remarks, "he 

 certainly never before heard that the vapour -density of any given 

 body pointed to some particular number as the expression of 

 its molecular weight." This is an extraordinary admission. 

 Is it possible that the first and most obvious consequence 

 of the equality in bulk of gaseous molecules is a sealed book 

 to Mr. Odling's school ? How powerful then must be the in- 

 fluence of preconceived notions when they shut out from intel- 

 lectual vision the almost self-evident fact that vapour-densi- 

 ties must be proportional to molecular weights if molecules are 

 of equal bulk, and therefore that they (the vapour-densities) 

 point directly to, or in fact actually express, the molecular 

 weights. If it is not self-evident, here is the proof: — 



Let w— weight of a molecule of the gas A, and W= weight 

 of a molecule of the gas B. If there are n molecules of the gas 

 A in the space of a cubic foot, there are n molecules of the gas B 

 in the space of a cubic foot. The weight of a cubic foot of gas 

 A is therefore nw, and the weight of a cubit foot of gas B is riW. 

 The vapour- density of gas A is to vapour-density of gas B as 

 nw:nW::w:W. Q.E.D. 



Molecular weights are ratios, vapour-densities are ratios ; and 

 these ratios are identical if all molecules in the gaseous state 

 have the same bulk. 



Mr. Odling may say that by vapour-density he means the 

 specific gravities of the vapours expressed in terms of water or 

 air as unity, and that these numbers do not point to " some par- 

 ticular numbers as the molecular weights : " but they certainly 

 do, inasmuch as molecular weights are ratios which are indicated 

 or " pointed to" by the ratio of the numbers that express the 

 vapour-densities. That the density of the gas of least specific 

 gravity should be selected in preference to any other as the 

 standard unit of comparison is clear from this, that it is easier 

 for the mind to compare magnitudes together when they are 

 expressed in terms of the least ; because if they were expressed 

 in any other, we should have to compare multiples with recipro- 

 cals, reciprocals with reciprocals, and multiples with multiples; 

 whereas expressed in terms of the least, we have to compare 

 multiples with multiples only — one mental process in place 

 of three. 



The specific gravity of gases and vapours in terms of hydrogen 

 unity are identical as numbers with the molecular weights ex- 

 pressed in terms of hydrogen unity. This Mr. Odling's school 



