406 Proceedings of the Royal Society 
authorities in chemical physics ; and among those who acknow- 
ledged the reality of the deviation there was irreconcilable dis- 
agreement as to the temperature at which in the descending 
scale the deviation begins. Hope in 1804 settled both points 
by a set of admirably contrived experiments, in which he took 
the temperature in various parts of a long column of water, — at 
the bottom, at the middle, and near the surface, — cold or heat 
being applied variably at these several points to produce the 
necessary intestine movements among the particles of the water. 
The whole paper on this subject is an admirable example of 
experimental reasoning, which definitively settled, as we now 
know it, a great fact in nature’s laws of much practical value in the 
economy of this earth, relative to the freezing and thawing of 
water, and the influence of these changes in its condition on the 
air, the land, and animal life. This was the second of Hope’s 
original researches, and the last ; and again our astonishment is 
raised that it should have been the last. 
In the highest walks of Mathematics and Physics, the Proceed- 
ings of this Society have always abounded with important investi- 
gations. No one can doubt that such must have been the case 
when Kobison, Playfair, Leslie, Ivory, and Wallace took a large 
share in the Society’s business. But this is a branch of my sub- 
ject to which I cannot myself do full justice; and time has not 
sufficed for me to complete my undertaking by asking aid from my 
well-qualified colleagues, who, I am sure, would have cheerfully 
granted me their assistance. The papers are chiefly researches in 
geometry and algebra of the most abstruse kind. But there are 
two of a different stamp — on the confines between chemistry and 
physics — which may be here shortly noticed, as they are the work 
of one whose labours will call presently for a larger share of our 
attention. 
In 1794 Dr Hutton communicates a dissertation “ On the Philo- 
sophy of Light, Heat, and Fire.” In this inquiry he reasons 
against the existence of radiant heat, apart from light, which had 
been announced recently before as an important discovery by 
Saussure and Pictet. Hutton maintained that the real radiation 
is of “ invisible light” — light so faint as not to be cognisable by 
