VOL. XLIV.] FHILOSOPHICAL TRANSACTIONS. 3QQ 



become more dense by the diminution of heat, which is the presence of cold: 

 and these alterations are always more or less sensible as the bodies are more or 

 less dense. 



The air we live in,' as it is the most rare and light fluid, so are its alterations 

 the most sensible; and indeed he knows of no experiments which deter- 

 mine how far it is capable of being expanded by heat, or condensed by cold; 

 only we find that it will make its way through any fluid in which it lay dormant, 

 when its elastic property is rouzed by the approach of such a heat as will make the 

 fluid boil. On the other hand, when compressed by a fluid so contracted by 

 cold as to freeze, or become solid, its elasticity will only bear a certain degree of 

 compression, till the force with which it endeavours to restore itself, exceeds the 

 force by which the parts of the solid, that confines it, adhere to each other, and 

 so bursts its prison ; as we often see during hard frosts in ice, and likewise glass, 

 and other hard bodies, whose parts cannot stretch. 



Next to air is alcohol, or the highest rectified spirits of wine: this and water, 

 and all other liquids, are capable of receiving no greater degree of heat than what 

 makes them boil, as was first demonstrated by M. Amontons; but that inge- 

 nious inventor of the quicksilver thermometer, Mr. Fahrenheit, has discovered, 

 that when the barometer marks a greater pressure of the atmosphere, the same 

 liquor will receive 8 or Q degrees more of heat, than when the barometer is at 

 the lowest. Hence Boerhaave gives the hint, that from nice experiments being 

 made of the different degrees of heat, marked by a thermometer in boiling water, 

 compared with the different heights of the barometer, and tables formed on them, 

 a thermometer applied to boiling water might, at sea, where the motion of the 

 ship hinders observations with the barometer, serve to determine the difference in 

 the gravity of the atmosphere. 



These, and all other liquids, by a certain determinate degree of cold peculiar 

 to each sort, lose their fluidity, and freeze, or become solid, but not in the same 

 order as by heat they boil ; for, by cold, oil or water is sooner frozen than spirit 

 of wine, though this will boil sooner than oil or water. All solid bodies like- 

 wise, as minerals, metals, and even stones, will become fluid, or melt, at a cer- 

 tain degree of heat peculiar to each species; and, when thoroughly melted, it is 

 probable they are capable of receiving no higher degree of heat; and, on the 

 absence of that heat to a certain degree, they all return to their natural solid state. 

 Hence we may reasonably conclude, that solidity is the natural state of all bo- 

 dies; and that some are only accidentally fluid, because their constitution is such 

 as to melt by those degrees of heat which our atmosphere is most commonly 

 subject to. All solid bodies are observed to contract into smaller dimensions by 

 cold, and gradually to expand at the approach of heat, till at last, being by heat 

 forced to the greatest degree of expansion, the particles of which they are com- 



