Prof. Odling on Chemical Notation. 121 



school," I answer that it is (as Mr. Waterston perfectly well 

 knows) quite impossible that such a consequence should be con- 

 cealed from any one, even if it had not been brought into promi- 

 nent notice by Ampere nearly half a century ago. But I am 

 still unconvinced that the numerical expression for the molecular 

 weight of any particular body is an abstract unchangeable ex- 

 pression, and not a relative one varying with the standard of 

 comparison adopted, — that the molecular weight of water, for 

 instance, will not be 9, or 18, or 11 2*5, accordingly as the mole- 

 cular weight of hydrogen is taken as 1 (Dal ton), or 2 (Laurent 

 and Gerhardt), or 12'5 (Berzelius). Mr. Waterston inconside- 

 rately reproached me with having represented the molecular weight 

 of water by the number 18 in opposition to its vapour-density, 

 whereas my argument was that if the molecular weight of hydro- 

 chloric acid is 36*5 (and by consequence the molecular weight 

 of hydrogen 2), then upon chemical grounds the molecular 

 weight of water will be 18 in accordance with its vapour- deusit}', 

 and not 9 in opposition thereto. Dalton and Berzelius alike 

 represented the ratio of the molecular weights of water and hy- 

 drochloric acid as being ■—, the ratio of their vapour-densities 

 being ^. Gerhardt first recognized the admittedly true ratio 

 of the molecular weights of water, hydrochloric acid, and hy- 

 drogen, and expressed it by the numbers 18, 36-5, and 2, which 

 Mr. Waterston in his grand scheme of reform proposes to 

 alter into 9, 18"25, and 1. He has absolutely nothing what- 

 ever to tell us about the molecular correlation of the bodies 

 represented, but simply wishes to halve their accepted volumes, 

 weights, and formulae ; which halving was considered long ago 

 and not accepted, because any abstract advantages that it might 

 possibly offer were not thought of sufficient consequence to coun- 

 terbalance the practical inconveniences which would accrue from 

 its adoption. 



Although the determination of the ratio of the molecular weights 

 of water and hydrate of potassium (or, in deference to Mr. Wa- 

 terston, hydrate of potash) is considered by chemists to be a 

 point of fundamental importance, and although the majority of 

 them have come somewhat unwillingly to the conclusion that 

 one molecule of water will form two molecules of the hydrate, 

 yet Mr. Waterston, contemptuous of all that has been done or 

 thought on the subject, boldly proclaimed in his first letter, 

 " Now the fact is that two molecules of water are engaged in 

 forming one molecule of the hydrate;" and the fact being thus 

 declared, all discussion upon the point was of course at an end. 

 But in his second letter he substitutes for his assertion of the 

 alleged fact an argument which he considers to establish the 



