June, 1905.] 



KiNOWLEDGE & f SCIENTIFIC NEWS. 



127 



Or again, we take the same substance in a definite 

 locality, but in more tlian one chemical state, e.g., iodine 

 and silver, at first uncombined and then in the state of 

 combination. The questions to be asked are : Is the 

 weight conserved during the reaction ? Is the mass con- 

 served ? Is the acceleration g conserved ? 



The question to which most attention has been given, 

 though not always with a clear perception of the issues, 

 IS, Does chemical change influence weight ? 



It is easy to produce a chemical transformation in a 

 closed vessel, and tolerably easy to test on a balance the 

 weight of the contents both before and after the action. 

 Measures were made to this end in 1893 by Landolt on 

 (amongst other things) the combination of silver with 

 iodine, but with uncertain result. Later investigations 

 made by him (in 1900) on the transformation of a ferrous 

 into a ferric salt, in which the clearest evidence of 

 apparent weight was obtained, are unfortunately com- 

 plicated by the fact that there is here a change also in 

 the magnetic properties. A piece of iron placed in a 

 magnetic field becomes magnetised, and tends to move 

 in it unless the field is quite uniform. Any variation of 

 its magnetism due to a change of its magnetic perme- 

 ability would entail a corresponding change in the pull 

 from magnetic causes ; and this action might conceivably 

 be the cause of the apparent change in weight. Besides, 

 there might be a more direct connection between mag- 

 netism and gravity, so that change in one necessarily 

 provoked a [change in the other, whether the magnetic 

 field were uniform or not. It is interesting to be re- 

 minded that Faraday in 1850 had sought for a connec- 

 tion between electro-magnetism and gravity. He had a 

 " long and constant persuasion that all the forces of 

 Nature are mutually dependent " and although his 

 experiments led to a negative conclusion, yet the results 

 did not shake his " strong feeling of a relation between 

 gravity and electricity, though they give no proof that 

 such a relation exists." Leaving aside some unsatis- 

 factory experiments of Sanford and Ray in America this 

 was the state of affairs in 1901 when Heydweiller, of 

 Breslau, published in "Drude's Annalen " the results of a 

 series of experiments. These were made with every pre- 

 caution, employing a variety of reactions ; and in every 

 case but two a diminution of weight was found to have 

 occurred during the chemical change. The total weight 

 of reacting substance varied from 160 to 280 grammes, and 

 the alteration in weight amounted in one case to more 

 than one-fifth of a gramme. Excluding all those cases in 

 which the observed change did not exceed, or barely 

 exceeded, the expected errors inevitable to the experi- 

 ments, Heydweiller considers that an alteration of weight 

 has been safely established as taking place [a) when iron 

 reacts on alkaline or acid (but not neutral) copper sul- 

 phate solution, ih) during the solution of acidified copper 

 sulphate in water, and [c) during the action of caustic 

 potash on copper sulphate. No conclusion could be 

 arrived at as to the dependence of the change upon the 

 amount of action taking place. Nor does there seem to 

 be any obvious reason why alkaline and acid solutions 

 should exhibit a different behaviour from a neutral 

 one if the change in apparent weight is in reality due to 

 an alteration in gravitational pull. It should be observed 

 that the reactions employed are only mild ones. The 

 impossibility of employing more vigorous ones arises from 

 the necessity of preventing any action from taking place 

 until after the first weighing. The transformation took 

 place in an inverted U-tube, each limb at first containing 

 one of the substances that were afterwards to be mixed. 

 And, indeed, Lord Rayleigh has pointed out that a pos- 

 sible source of error in the experiments is that, even with 



the materials actually used, some change may have been 

 progressing during the first weighing. If, for example, 

 copper sulphate is in one limb and water in the other, 

 there will not be complete equilibrium; water will distil 

 over to the salt, and although this motion will not directly 

 modify the pull on the balance, since the forces called into 

 play are internal forces, yet thermal change will accom- 

 pany the evaporation and condensation of the water (the 

 hmb containing the water will cool and the other will 

 rise in temperature), and the difference of temperature 

 thereby set up will interfere with the accuracy of weigh- 

 ing owing to the con\ection currents that it will produce. 

 It must be remembered that the effect observed is only 

 small, and although every endeavour was made to ex- 

 clude possible sources of error, it must be admitted that 

 the results form a very precarious foundation for theory. 

 The evidence would be strengthened if there were some 

 degree of regularity in the amount of the change ; but no 

 regularity exists apart from the fact that the change is 

 negative in all the cases in which it is greater than the 

 expected error. The magnitude of the change observed 

 is well within the powers of a good balance to demon- 

 strate ; there is therefore every reason to hope that by 

 the accumulation of evidence all doubt will eventually be 

 removed. 



It would be of great theoretic importance to learn that 

 some change does really occur. At present, gravitation 

 is somewhat of a stumbling-block from the point of view 

 of theory. It is so indifferent to circumstances. How is 

 it that the earth pulls a body with sensibly the same force 

 whether a plank (say) is interposed or not ? What is the 

 nature of this tie between the two bodies which is not 

 severed thereby ? It is true that by inserting a 

 plate of a magnetically indifferent body like copper 

 between two magnets the attraction (or repulsion) 

 between them is not modified. But in what sense can 

 we regard a plank as being gravitationally indifferent ? 

 Again, it has recently been shown by Poynting that the 

 pull on a crystal such as quartz, which is a substance that 

 in most respects exhibits different physical properties in 

 different directions, does not sensibly depend upon the 

 orientation of it with respect to the earth. Whether it 

 will turn out or not that the ether is the medium con- 

 cerned with the transmission of gravity, it is clear that 

 the propagation takes place in practical independence of 

 the structure of the matter through which it passes. 

 From the general physical behaviour of bodies, therefore, 

 it is not to be expected that any modification should be 

 brought about by chemical change which, as far as we 

 know, simply consists in a re-arrangement of the finer 

 parts of which a substance is composed. Any modifica- 

 tion, then, which may eventually be demonstrated to take 

 place will introduce a new element into the theoretic 

 consideration of gravity. It will indicate that, from the 

 point of view of this question, the nature of the changes 

 which are dealt with in chemistry are of an essentially 

 different type from the coarser changes which are termed 

 physical. Mixing two substances together may not 

 change their weight, and, in view of Poynting's experi- 

 ment, we do not expect that it will ; but bring about the 

 more intimate chemical union, and the grip of the earth 

 on the body may have changed. The knowledge gained 

 by the final settlement of this question will affect not 

 only gravitational theory, but will have to be taken 

 account of also in the consideration of the exact nature of 

 the forces which come into play in chemical change. 

 On the other hand, if it should turn out that the varia- 

 tions found by Heydweiller are due to unsuspected sources 

 of error, there will be no fact known connecting gravita- 

 tion with any other physical property of matter. 



