190 Mr. Rainy's Reply to Dr. Thomson, [MahcHj 



the experimental result to vary from 1 6 by more than 0"5, it 

 must approach some other integer. The variation from the 

 theoretic result in this experiment is actually about one-fourth 

 of the greatest possible variation. Dr. Thomson would probably 

 reply, that the greatest possible variation would be 1 and not 0*5, 

 if we compare the atomic weights of oxygen and hydrogen, 

 instead of their specific gravities ; but even admitting this, the 

 deviation from the theory is still about one-eighth of the greatest 

 possible deviation. All numbers which have large ratios to any 

 given number must of course nearly coincide with integral multiples 

 of that number, and the coincidence must be so much the nearer^ 

 as the ratios which the former have to the latter are larger. This 

 is precisely the case with hydrogen. Its atomic weight com- 

 pared with other bodies is extremely small, and, therefore, if we 

 assume it as unity, the atomic weights of other bodies must 

 nearly be integers. Were astronomers to adopt the mass of the 

 moon as unity, they might represent the masses of the primary 

 planets by integers, with great precision, yet I suppose no one 

 has ever fancied that the latter were exact integral multiples of 

 the former. Dr. Thomson will himself admit that the truth of 

 the hypothesis could scarcely be ascertained, were the proofs to 

 rest on a direct comparison of the atomic weights of mercury, 

 iodine, or even chlorine, with that of hydrogen, because even 

 the lightest of these atoms, that of chlorine, is about 36 times ets 

 heavy as the atom of hydrogen. It would not be at all surpriz- 

 ing, from the imperfections to which even the most refined 

 chemical researches are still liable, that the actual number 

 should differ by more than even a whole unit from the above 

 number. Davy's experiments led him to adopt a number which 

 on this scale would be 33^. I believe Dr, Thomson's number 

 to be nearer the truth ; but how are we to decide whether the 

 true atomic weight is an integer, when two sets of experiments 

 performed with great care indicate a difference of two units and 

 a half in the result. To ascertain the truth of the theory, we 

 must have recourse to atoms of which the weights do not bear 

 such a large proportion to that of hydrogen. Such are the atoms 

 of carbon, oxygen, and azote. Could it be proved clearly that 

 there are integers on the hydrogen scale, there would be strong 

 reasons for believing that the other atoms are also integers. 

 This might at first appear a simple task; but we must remember 

 that the object in view is not to discover the integer which most? 

 nearly coincides with the true number ; but to discover whether 

 or not the true number be precisely an integer, and not a fraction. 

 The method of ascertaining the relative atomic weights from 

 the relative specific gravities of the gases, appears to afford by 

 far the most accurate results ; and yet from tlie impurity of the 

 gases, and from the small quantities that can be weighed, it is 

 extremely difficult, if not impossible in the present state of the 



