140 Correspondence — J. V. Elsden — J. R. Dakyns. 



are of the utmost importance from a geological standpoint. That 

 crystalline affinity is a definite molecular force accompanied by 

 exothermic changes is doubtless correct, but whether this force 

 can determine chemical changes of an endothermic character is 

 a question involving an entirely new conception, and requires 

 careful consideration before it can be accepted as a reasonable 

 hypothesis. More especially is this the case when we have to deal 

 with the constitution of hydrates, which afford such excellent 

 examples of the application of the phase rule in chemical physics. 

 Hydrates, as is well known, have a vapour pressure of their own, 

 and only continue to exist when in equilibrium with tbe vapour 

 pressure which they have to support. Thus the hydrates of copper 

 sulphate can be successively decomposed under varying conditions 

 of temperature or pressure. The instability of the aluminium 

 hydrate, Alj O3 . 3 Hj 0, at moderate temperatures, also, is a fact 

 well known in chemistry ; and it seems probable that the occurrence 

 of any hydrate, either of aluminium or of iron, in nature will 

 depend upon which happens to be the stable phase under the 

 existing conditions of temperature and pressure. 



In connection with the crystallization of alumina the researches 

 of W. Spring, of Liege, appear to have some bearing. According 

 to this observer, amorphous alumina or ferric oxide, if damp, can 

 be rendered compact, presumably with the occurrence of an incipient 

 crystallization, by pressure alone; and we are induced to consider 

 whether the amorphous state in solids may not, in some cases at 

 least, be comparable with the condition of superfused solutions and 

 glasses. In fact, many of the distinctions between solids and 

 liquids are gradually breaking down under the researches of 

 modern physics. 



There does not, therefore, appear to be any necessity for a new 

 theory to explain the facts in this case. It appears rather that 

 Mr. Holland has unnecessarily introduced a difficulty by presupposing 

 that the molecules in laterite are isolated from extraneous energy. 

 If this were really the case there could be no change of entropy 

 such as he describes, and rightly, to be the result of the reactions 

 involved. J. Vincent Elsden. 



38, St. Stephen's Gardens, Twickenham. 



THE COLOUR OF GLASLTN AND OF LLTN LLYDAW. 

 Sir, — Glaslyn and Llydaw are the names of the two chief 

 Snowdouian tarns. Glaslyn has been noted from time immemorial 

 for the greenish colour of its water, as is implied by its name ; but 

 until the Summer of 1899 there was nothing peculiar about the 

 colour of Llydaw. During that Summer, however, for the first time 

 within the last fifty years at least, the water of Llyn Llydaw became 

 as green as that of Glaslyn. The cause of this remarkable change 

 of colour is not far to seek ; for in the Spring of 1899, some time 

 about March I am told, the company that works the Snowdon 

 Copper Mine commenced crushing and washing their ore on the 

 bank of Llydaw, so that a large quantity of greenish debris was 



