282 SCIENTIFIC AMUSEMENTS. 



it flows, and the actual current or quantity of water passing through 

 the pipe in a given time. In like manner, the phenomena of heat, 

 when considered to the fullest extent, involve an analogous pair 

 of notions, viz., the degree of manifestation of heat termed 

 temperature, to some extent correlative with the pressure of the 

 water current ; and the quantity of heat present, corresponding 

 with the quantity of water flowing. Similarly, in relation to 

 electric current energy, two analogous ideas are requisite, viz., that 

 of potential or tension, and that of quantity of electricity. 



Variations of temperature are indicated by instruments termed 

 thermoscopes, and are usually measured by means of thermometers 

 (such as the mercurial thermometer, Expt. 28), dependent for 

 their indications on the alteration produced in the bulk of a 

 given substance or object by its expansion and contraction as the 

 temperature varies, the starting point being determined by some 

 well marked natural change, always produced at one and the same 

 temperature, such as the melting of ice or boiling of water under 

 definite conditions. The quantity of heat, however, cannot be 

 thus measured ; for it is found that very different results are ob- 

 tained with different substances under the same conditions ; and 

 therefore some one substance must be selected as a standard. 

 Thus, suppose two precisely similar glass flasks to be heated in 

 exactly the same way over a lamp or other source of heat ; in one 

 water is put, so that the effect of the passage of the heat into the 

 water by conduction through the walls of the flask is to heat up 

 the water, i.e., to cause its temperature to rise. Suppose some 

 other fluid, such as mercury, to be placed in the other flask and 

 this to be heated in the same way ; obviously the rate at which 

 heat passes through the walls of the two flasks will be the same ; 

 but, nothwithstanding, it will be found that the mercury heats far 

 more rapidly than the water if equal weights of the two be used ; 

 whilst if the quantities of water and mercury are so adjusted that 

 they both rise in temperature at the same rate (say 5 per minute, 

 or so) it will be found that the mercury weighs about 30 times as 

 much as the water. This difference is expressed by saying that 

 mercury has a much smaller capacity for heat than water ; so that 

 a given quantity of heat will produce some 30 times the effect in 

 heating up mercury that it will when made to heat up an equal 

 weight of water. 



Expt. 328. To show that the Human Body will not always 

 distinguish Temperature accurately. At first sight it would 

 seem that nothing is more simple than to distinguish heat from 

 cold by the sensation produced on the body ; but in point of fact 

 such is not the case. In the depth of winter in the Arctic regions 



