at the galvanometer. It begins to rise !< >\vly, slowly, 



and stops, instead of at 50, at 8. Why is this ? We cannot 

 as yet the reason sec! The Iron being the best conductor 

 of heat, its molecules >nH'U fin Fig. 15), according to the 

 theory advanced, strike the molecules of ZA, and generate 

 a greater I-'.. M I as the consequence. How can we get at 

 the truth of this matter with I ng us in the 



With regard to the molecules of bodies, three different 

 ttion present thcmse'.\ First, the solid 

 nd, the liquid or fluid state; third, the 

 i- experiments have only to do with the first or the 

 solid state, although, of course, the two latter come into 

 play in many of the other sources of electricity. From 

 considerations based upon various physical phenomena, 

 Sir W. Thomson has calculated, that, in ordinary solids 

 and liquids, the average distance between contiguous mole- 

 cules is less than one hundred millionth, but than 

 the one two thousand millionth, of a centimetre ; and to 

 form an idea of the size of the molecules, Sir W. Thomson 

 gives this illustration : "Imagine a drop of rain, or a glass 

 " sphere, the size of a pea, magnified to the size of the 

 "earth, the molecules in it being increased in the same 

 " proportion. The structure of the mass would then be 

 " coarser than that of a heap of fine shot, but probably not 

 " so coarse as that of a heap of cricket balls." Professor 

 Tait, when making the same calculation, suggested marbles. 

 The number of molecules in a cubic inch of gas at o*C. tem- 

 perature and 760 millimetres pressure, has been calculated 

 to be about one hundred thousand million million million, or 

 ioo,CKX),coo,CKX),coo,oc)o,0(X),ooo (1023). The molecules of 

 a gas are always in a rapid motion. The mean velocity of 

 hydrogen molecules at O'C. and 760 millime: about 

 6,000 feet, or i/7 miles per second (the length of its path 

 being .0,001,855 millimetres, and nine thousand four hundred 

 and eighty millions [9,480,000,000] for the number of its 

 impacts). 



