conformity with the Dynamical Theory of Heat and Gases. 253 



called hypothesis that all simple gases contain equal numbers of 

 atoms in equal volumes, it does not recognize the difference 

 between gaseous molecule and chemical atom so distinctly as to 

 avoid obscurity and misrepresentation : e. g. Mr. Odling states 

 that "when water is acted uponby metallic sodium, one equiva- 

 lent of sodium expels one equivalent of hydrogen to form the 

 well-defined body hydrate of sodium, and the action proceeds no 

 further ; but when melted hydrate of sodium at a temperature 

 approaching that of redness is acted upon by metallic sodium, a 

 second equivalent of sodium turns out a second equivalent of 

 hydrogen to produce oxide of sodium." The diagram appended 

 represents the half of the hydrogen in one molecule of water as 

 expelled by a molecule of sodium to form the solid hydrate, and 

 afterwards the second half to produce the oxide of sodium. Now 

 the fact is, that two molecules of water are engaged in forming 

 the one molecule of the hydrate, and the hydrogen of only one 

 of them is expelled in forming the hydrate ; and in the terminal 

 action, the hydrogen of the other molecule of water is expelled 

 by a repetition of the same action between the newly supplied 

 sodium and the oxygen of that second molecule. 



Thus also Dr. Graham, in his second volume, p. 514, states 

 with reference to the analogous action between potassium and 

 water, " The representation of water by HHO, as in Gerhardt's 

 system, possesses the additional advantage of corresponding 

 with the important fact that it is possible to replace either the 

 half or the whole of the hydrogen in* water by a metal. Thus 

 potassium thrown into water displaces half the hydrogen and 

 forms hydrate of potash, HKO; and when this compound, in 

 the solid state, is heated with an additional quantity of potas- 

 sium, the remaining half of the hydrogen is displaced, and 

 anhydrous potash, KK9, is formed. On the contrary, when 

 potassium acts on hydrochloric acid, HC1, it displaces the whole 

 of the hydrogen and forms chloride of potassium, KC1. This is 

 an important difference, which is easily understood on the sup- 

 position that water contains two atoms, and hydrochloric acid 

 only one atom of hydrogen ; whereas if these two compounds 

 are represented by the analogous formula? HO and HC1, the cause 

 of the difference of action is by no means apparent." The am- 

 biguity here is caused by Gerhardt's symbols actually represent- 

 ing two gaseous molecules ; but they are not distinctly recognized 

 as having this meaning either by Mr. Odling or Dr. Graham, as 

 the following shows. In the above extract HKO represents one 

 molecule of the hydrate of potash, but HHO represents two mole- 

 cules of water, and KKO two molecules of anhydrous potash. The 

 actions expressed by the fractional system of notation (submitted 

 * Here is an ambiguity. Is one molecule meant, or two ? 



