:-)90 Prof. H. E. Armstrong on Low-Temim'ature Research 



given be in any way correct ; colour might arise, however, if several 

 such molecules were so juxtaposed as to give rise to a system of 

 junctions similar to those which appear to condition the appearance of 

 colour in hydrocarbons and other compounds. The manner in which 

 the colour of water changes as its state is varied should therefore afford 

 some insight at least into the direction in which changes in structure 

 are proceeding. 



Tlie alteration in colour from scarlet to yellow in the case of 

 vermilion and of mercuric iodide and from yellow to white in the 

 case of ui-anic nitrate and of ammonium platinum chloride, which 

 is effected on cooling these substances in liquid air, may be 

 regarded as affording proof of molecular simplification consequent 

 on the suspension of unions of the residual affinities such as are 

 referred to on p. ?u^. In the case of organic colouring matters, in 

 which colour is conditioned by peculiarities of structure within the 

 fundamental molecules, cooling has little if any influence on the 

 colour. 



It is when these various applications of the knowledge which is 

 being gained of the properties of matter at low temperatures are 

 appreciated, that the work done in the Royal Institution Laboratory 

 is seen to be of such exceptional importance. 



The determinations which have been made of the densities of 

 oxygen, nitrogen and hydrogen in the solid state show that in these 

 cases the density increases as tlie liquid becomes solid. It would be 

 a matter of great interest if tlie comparison which can be made in 

 the case of water lietween the solid and liquid states could be extended 

 to other substances which are condensed only at low temperatures ; at 

 present the density of the solid in comparison with that of the liquid 

 is known in but a few instances. 



Sir James Dewar has calculated from his determinations of their 

 densities at low temperatures that the molecular volume in the solid 

 state — the volume occupied by the molecular proportion in grammes — 

 of oxygen, nitrogen and hydrogen would ])e 21*2, 25' 5 and 24*2 

 cubic centimetres at the absolute zero. It is interesting to note 

 that the molecular volume of liquid hydrogen at its boiling point is 

 28*6, whereas that of liquid helium is 26 '6. It is remarkal)le that 

 the values should be so nearly alike, taking into account the great 

 difference in the masses of the molecules. 



India-rvl)her at Low Temiieratares. — Perhaps the most striking 

 case of alteration in properties effected by extreme refrigeration is 

 afforded by india-rubber, which is one of the most elastic of sub- 

 stances at ordinary temperatures : when cooled in liquid air. however, 

 it becomes so rigid and brittle that it is easily pulverised. Films 

 1/50 mm. thick are no longer permeable by liquid oxygen, or even 

 liquid hydrogen. The main constituent of india-rubber is a complex 

 hydrocarbon of the turpentine class, containing a number of ethenoid 

 junctions ; these junctions presumably determine its peculiar physical 



