646 



LECTURE LII. 



ON THE MEASURES AND THE NATURE OF HEAT. 



XhE principal particulars concerning the origin, the progress, and the effects 

 of heat, having been noticed in the last lecture, we now proceed to examine 

 the most usual modes of measuring its degrees and its quantity,and to inquire 

 into the most probable opinions respecting its intimate nature and its imme- 

 diate operation. 



The expansion of solids is measured by a pyrometer, which is calculated 

 for rendering the smallest change of dimensions perceptible either by me- 

 chanical or by optical means. The first of these methods was adopted by 

 those who first investigated these ejects; a bar of metal being placed in a 

 vessel of water, or of oil, which was heated by lamps, while the extremities 

 of the bar were in contact with a fixed point on one side, and on the other 

 with a series of levers, which multiplied the expansions, so as to render them 

 easily observable by means of the end of the last lever, serving as an index. 

 But it is obvious that the expansion of the fixed part of the instrument, and 

 the irregular changes of temperature of the levers themselves, must very 

 much interfere with the accuracy of such an instrument. A much more cor- 

 rect mode of determination is to employ two microscopes, fixed to an appara- 

 tus, which is always kept, by means of ice, at a constant temperature, and to 

 observe with a micrometer the change of place of either end of the heated 

 bar. - • 



For such purposes, the degrees of heat may be ascertained by the natural 

 measures of the freezing and boiling points of certain liquids, and of water in 

 particular; but for subdividing the intervals betM'een these points,, other 

 means must be employed. The most natural mode o^determining the inter- 

 mediate degrees of heat, which must be considered as the standard for the 



