286 PROFESSOR LETTS ON PHOSPHORUS-BETAINES. 
and this fact, considering the similarity of the two elements, renders it probable 
that at a sufficiently high temperature the molecules of phosphorus would 
behave in a similar manner.* 
Regarding other physical properties of the three elements, such as specific 
gravity, atomic volume, &c., it is not necessary to say much, as nitrogen, on 
account of its gaseous nature, does not admit of a ready comparison with the 
other two. It may be mentioned, however, that both the atomic weight 
and specific gravity of phosphorus and sulphur are very close to each other, 
and consequently their atomic volumes are nearly identical.+ 
Turning now to the chemical properties of the three elements (in the free 
state), we again find a close similarity between phosphorus and sulphur, 
Whereas nitrogen possesses scarcely a point of resemblance to either; for 
whilst the former are characterised by their energetic attraction for other 
elements, nitrogen is strikingly ert, and displays scarcely any tendency to 
enter into combination. 
The great affinity of phosphorus for oxygen needs no comment; that of 
sulphur for the same element is considerably less, but is still well marked ; 
whilst nitrogen possesses so slight an attraction for oxygen, that its oxides are 
powerful oxidising agents. We have then in phosphorus, sulphur, and nitrogen 
a group of elements which show a regular gradation in affinity for oxygen ; 
and, as we might expect, the affinity of these elements for hydrogen is in 
exactly the reverse order, ammonia being the most stable of their hydrides, 
and phosphuretted hydrogen the least, whilst sulphuretted hydrogen stands 
midway between them. We might perhaps expect from these facts that, as 
ammonia is the most alkaline of all the hydrides, sulphuretted hydrogen would 
be more alkaline than phosphuretted hydrogen; but this is not the case, for the 
latter has a neutral reaction, and combines directly with hydriodic and hydro- 
bromic acids, whereas sulphuretted hydrogen has a slight, but still a distinct 
acid reaction, and does not, so far as we know, combine with any hydracid. 
The difference observed in the affinity of phosphorus, sulphur, and nitrogen, 
for oxygen and hydrogen, exercises, as we might expect, an important influence 
on the properties of their compounds. Thus most compounds of phosphorus, 
with electro-positive elements or compound radicals, oxidise spontaneously, as 
in the case of phosphuretted hydrogen, many metallic phosphides, and the 
* The author has communicated with Professor Vicror Mryer on this subject, who stated that he 
had already made experiments in this direction, and that they indicated a diminution in the vapour 
density of phosphorus at a high temperature. Professor Mnyrr having thus established his priority to 
any experiments on the vapour density of phosphorus at high temperatures, the author has left the 
matter in his hands. 
+ According to Ramsay (Journ. Chem. Soc., 1879), the sp. gr. of sulphur at its boiling-point is 
1:4799, and its atomic volume (in the sense in which Korr employs the term) 21:6. The same author, 
in conjunction with Masson (Journ. Chem. Soc., 1880), gives the sp. gr. of phosphorus at its boiling- 
points as 14850, and its atomic volume as 20°91. 
