Mr. M. M. Pattisou Muir o;i Chemiml Classijication, 99 



istence of a niolecnle NH4 CI, must we ac/ree with those who 

 look on sal ammoniac as a " molecular " compound of am- 

 monia and hydrochloric acid? I do not think that it is neces- 

 sary for us to do so. 



The difficulties Avhich must he overcome by the theory 

 of molecular compounds are numerous. First of all there 

 is the great difficulty of defining the expression " molecular 

 compound." How is a compound of this class to be dis- 

 tinguished from an atomic compound ? I do not think I 

 am wrong in saying that no one has as yet given a satis* 

 factory answer to this question. Is the mere fact that a 

 compound is dissociated by heat sufficient to prove that the 

 structure of the compound is different in kind from that 

 of stable bodies? Many substances which are dissociated 

 by heat are nevertheless possessed of well-marked chemical 

 and physical properties which are not the mean of the pro- 

 perties of their constituents. It is difficult to believe that 

 the force holding the parts of these bodies together is essen- 

 tially different from that which acts between the parts of 

 other bodies perhaps only differing from the first in that 

 they are capable of existing as gases. It may be, however, 

 that in so-called ^^ atomic " and "molecular" compounds 

 we have the initial and final members of a series of bodies 

 gradually passing into one another by almost insensible de- 

 grees. Even assuming this difficulty to be overcome, there 

 remains the fact many gaseous (that is, by hypothesis, atomic) 

 compounds are decomposed at high temperatures. Water is 

 resolved into its atoms by the action of great heat. Where 

 are we to draw the line and say, all substances stable imme- 

 diately above this point, although decomposed at considerably 

 higher temperatures, are atomic compounds, but all substances 

 decomposed below this point are only molecular compounds ? 

 What are we to say to such substances as methylic and 

 ethylic sulphates ? (C2 11,5)2 SO4 cannot be obtained as a gas 

 without suffering decomposition, (0113)2 SO4 is stable in the 

 gaseous state. 



Although we may not agree with the theory of molecular 

 compounds, there is yet one lesson which it teaches and which 

 we should surely be ready to learn — namely, that in all dis- 

 cussions as to the valency of any element we can draw safe 

 conclusions only from a consideration of those compounds 

 which are capable of existing in the state of gases. 



[To be continued.] 



H2 



