XI—On Phosphorus-Betaines. By Professor Letts. (Plate X VIIL.) 
(Read January 3, 1881.) 
In a paper by Professor Crum Brown and the author on Dimethyl-Thetine 
and its Derivatives,* attention was drawn to the analogies which frequently 
exist between elements which have different atomicities, and which are usually 
considered as belonging to different families. The most striking examples of 
such elements are boron and carbon, gold and platinum, and phosphorus and 
sulphur. 
Since the publication of that paper, the author has pursued the subject, and 
his experiments, which have been made with the object of comparing the 
properties of analogous compounds of nitrogen, phosphorus, and sulphur, have 
confirmed the view that the two latter elements are very closely related, and 
that in many cases at least, phosphorus is more nearly allied to sulphur than 
it is to nitrogen. 
In the course of these experiments many facts and considerations relative 
to the three elements have occurred to the author, which he believes have not 
hitherto been presented in a clear and concise form. No doubt, some of them 
have been noticed by other chemists, but he believes that such has not been 
the case with all, and he is therefore induced to give a slight sketch of the 
analogies and differences which the three elements exhibit, before proceeding to 
describe his experiments. 
A Comparison of the Properties of Nitrogen, Phosphorus, and Sulphur. 
If we compare the three elements in the free state, we cannot but be struck 
with the very close analogies existing between phosphorus and sulphur, and 
the great dissimilarity of nitrogen to either. 
Phosphorus and sulphur are solid bodies ; both exist in allotropic modifi- 
cations which are produced by the action of heat on a particular form of each 
element. Nitrogen is gaseous, and so far as is known does not exist in 
more than one condition. 
Again, both sulphur and phosphorus have what is usually termed “abnor- 
mal” vapour densities; that is to say, in the gaseous state their molecules 
contain more than two atoms. At a sufficiently high temperature, however, 
the molecules of sulphur are dissociated into simpler ones containing two atoms, 
* These Transactions, vol. xxviii. 
VOL. XXX. PART I. 25 
