14 P ro f' Leslie on Heat and Climate. [July, 



manent condensation. Clay, for instance, is brought into that 

 state by burning, and the metals by hammering. By this 

 change of constitution, these bodies are rendered incapable of 

 retaining their natural share of heat. The particles of this fluid 

 have their attraction to the matter increased, but not in an 

 equal degree to that of their mutual repulsion. The relative 

 proportion of these forces, which may be called the specific at- 

 traction for heat, will, therefore, determine the quantities of 

 that fluid contained in equal communicating masses ; and an 

 equilibrium of temperature will obtain whenever the quantities 

 are as the specific attractions. The term capacity has been 

 employed to express the same property of bodies, but the idea 

 which it naturally suggests is perhaps inadequate. 



This train of speculations leads to a curious and important 

 fact, which is, that, if substances be ranged according to their 

 densities, their specific attractions for heat will in general 

 follow the inverted order. Thus when the temperatures are 

 alike, hydrogenous gas contains more heat than atmospheric 

 air, and both these fluids more than water* ; the quantity is still 

 smaller in the earths and stones, and very minute in the metals : 

 even among these the same principle may nearly be traced ; 

 there is a greater specific attraction in iron than in tin, in tin 

 than in lead, in lead than in mercury, and the oxides bear a 

 similar relation to their metals. There are a few exceptions, 

 however, to the general principle ; ice, for instance, has a smaller 

 specific attraction than water, and mercury than gold. Nor is 

 this to be wondered at ; for though the condensation of matter 

 tends to diminish that force, yet, as it results from the joint 

 action of all the elementary points, it must likewise be affected 

 by a change in their arrangement and constitution. If the pe- 

 culiar properties of a substance be not altered, the general prin- 

 ciple will invariably apply. Hence, the specific attraction of any 

 body for heat must perpetually fluctuate according to the de- 

 gree of dilatation arising from whatever cause. This fluctuation 

 is in most cases inconsiderable, yet appears to be one of the 

 chief sources of error in the solution of the curious problem of 

 finding the thermometric zero.f 



No substance more easily receives a change of volume than 

 air ; and its specific attraction for heat may, therefore, be pre- 

 sumed to be liable to great alterations. But to determine these 

 with accuracy is a very difficult investigation. 



The most obvious plan would be this : suspend in the receiver 



* Such was the opinion which then prevailed, Dr. Crawford having reckoned 

 the capacity of air near double that of water. But my hygrometrical researches, 

 some years afterwards, convinced me that it is six times less, not exceeding three- 

 tenth parts of the capacity of water. — A. 



t We may remark l)y the way that the principles already investigated show 

 that light is emitted with equal velocity from all luminous bodies; a fact which 

 theory might deduce from the common observation, that the rays of the same 

 species, from whatever object they are transmitted, suffer equal refractions on en- 



