TRANSACTIONS OF SECTION B. 581 



7. On the Nature of Solidion} By Professor Spencer U. Pickering. 



Although various special experiments seem to render it incontestable that 

 hydrates of a salt do exist in solution in some cases, the strength of the hydrate 

 theory should be derived from more general considerations as to the nature of the 

 solvent and substance dissolved, and from the thermal results of dissolution. At 

 the same time, however, equally general considerations render this theory inade- 

 quate to explain all the facts observed, though to such special lines of argument, 

 such as those which Dr. Nicol derives from the specific volumes, no weight what- 

 ever can be attached, for they would lead to the conclusion that the radicles com- 

 posing the salt itself are no more united than is the salt with its water. The 

 explanation of this conflicting evidence may be explained by the recognition of 

 the real complexity of the units of matter ; a great number of our so-called mole- 

 cules unite to form aggregates which bear the stamp of true chemical compounds 

 in most respects, but are so greatly Influenced by physical conditions that their 

 composition, to our imperfect means of investigation, appears indefinite. The 

 existence of such compounds alone can explain the peculiarities of minerals, 

 artificial crystals of isomorphous salts, alloys, basic salts, &c. Dissolution is a 

 result not of the formation of definite hydrates onlj-, but also of (apparently) 

 indefinite hydrates. The formation of these hydrates would always be accompanied 

 by an evolution of heat, but at the same time the aggregates of a solid being more 

 complex than those of a liquid, an act of decomposition absorbing heat would 

 also be a no less invariable accompaniment. The coexistence of two such counter- 

 acting actions can alone explain ail the thermal effects of dissolution. 



8ATUEDAY, SEPTEMBER 4. 

 The following Papers were read : — 

 1. On the Fading of Water-colours. By Professor W. N. Hartley, F.B.8.. 



Referring to the article Light and Water-colours, written by Mr. J. C. 

 "Robinson, in the ' Nineteenth Century,' Professor Hartley pointed out that colours 

 consist of mineral substances, for the most part of a stable character and organic 

 substances consisting of stable colours and unstable changeable colours. With the 

 exception of ultramarine, bodies of the former class may be generally considered as 

 unalterable unless they contain lead or mercury ; those of the second class may be 

 considered alterable under certain conditions. The action of light, according to the 

 researches of Chastaing, on these two classes of substances when it is capable of 

 affecting them is diflerent : on mineral substances the red rays cause oxidation ; the 

 oxidising power decreases as the rays tend more towards the yellow; the yellowish- 

 green rays are without action and form a neutral region ; the blue rays reverse the 

 action of the red — for they promote reduction — and this action increases in intensity 

 in the violet and ultra-violet rays. 



On organic substances the action of light is an oxidising one throughout, con- 

 tinuously increasing in power as the rays extend through the red and yellow into 

 the violet end of the spectrum. There is, however, a modification in the green, the 

 rays having a diminished oxidising power. The action of light on organic sub- 

 stances is not confined to oxidation, for bodies of a complex and imstable character 

 may be changed in composition, and, being resolved into more stable compounds, 

 changed in colour or rendered colourless. 



Experiment on colours showed that this was apparently the case with crimson 

 lake, gamboge, bistre, and to a less extent with brown madder and sepia. Indigo 

 is permament. Ultramarine is bleached by acids, but not by light. 



In order to preserve water-colour drawings in which yellow and delicate red tints 



■ Clwmical News, 54, p. 215. 



