AlcrsT 28, 1903.] 



SCIENCE. 



283 



ing amplitude because of the suspended 

 weights, so that the advancing wave train 

 leaves behind it an ever-lengthening tail, of 

 which the amplitude diminishes backwards. 

 The extreme head of the wave train travels 

 at what is called the wave velocity and the 

 middle of the spreading train travels at what 

 is called the group velocity. Strictly, the 

 term wave velocity applies to the ratio wave- 

 length divided by periodic time in the middle 

 region of a train of waves so long that the 

 diminishing amplitudes in head and tail are 

 without influence. 



It is a curious fact, as has been pointed out 

 by Heaviside, that a periodic wave train in a 

 dispersing medium is about the only kind of 

 wave that can be put into simple mathematics, 

 while a mere wave pulse is the only kind that 

 is simple physically. Physically a wave train 

 in a dispersing medium is a very complicated 

 phenomenon. 



VARIATION OF WEIGHT WITH CHEMICAL AND 

 PHYSICAL CHANGES. 



The electromagnetic theory of inertia, in 

 which the inertia of matter is attributed 

 to corpuscular electric charges in the struc- 

 ture of atoms, leads one to expect a decrease 

 in the total inertia of two substances like 

 H and O when they combine to form water 

 for the following reasons. A moving electric 

 charge has inertia. The amount of this 

 inertia is determined by the extent to which 

 the electric lines of force from the charge 

 permeate surrounding space, for this deter- 

 mines the extent of the magnetic field which 

 is produced by the movement of the charge. 

 Most of the inertia eflfect is, however, in the 

 region near the charge, for there the electric 

 field and also its magnetic eifect are greatest. 

 Two adjacent opposite charges side by side 

 have less electrical inertia than the same two 

 charges widely separated, for the reason that 

 the electric lines of force permeate less into 

 remote regions of space. 



If inertia and gravitation vary together we 

 should thus expect a given amount of O and 

 H to weigh less when these substances are 

 combined to form water. 



Very careful attempts have been made to 

 detect changes in weight due to chemical 

 changes by Landolt in 1893 and by Heyd- 

 weiller in 1900, and the changes are so small 

 as to be questionable. Attention was called 

 in ' Physics Notes ' several years ago in Sci- 

 KNCK to the fact that a variation of weight (or 

 mass) with chemical changes would by no 

 means necessarily vitiate the principle of the 

 conservation of matter, so that sucli changes, 

 if they exist, are of most importance in their 

 bearing upon the perplexing questions of 

 gravity and inertia. 



Recently it is announced that Professor 

 Babcock has established the fact of the varia- 

 tion of weight with chemical and physical 

 changes. He is reported to have used a spe- 

 cial form of hydrostatic balance capable of 

 detecting a change in weight of one part in 

 a hundred million. This degree of refinement 

 is in fact about that which can be reached by 

 the ordinary balance, and when we remember 

 that the temperature of his water-bath would, 

 unless compensating devices are devised and 

 used, have to be controlled to about 1/40,000 of 

 a centigrade degree to enable him to avail him- 

 self unmistakably of a sensitiveness of one 

 part in a hundred million, it seems doubtful 

 that he could have realized a sensitiveness 

 anything like as great as that at the dispogal 

 of Landolt in 1893, at the disposal of Heyd- 

 weiller in 1900 and also at any one's disposal 

 now in 1903. When the buoyant force of the 

 air, only, is involved temperature must be 

 controlled to about 1/400 of a centigrade de- 

 gree to enable one to detect unmistakably so 

 small a variation in weight as one part in a 

 hundred million. W. S. F. 



RESOLUTIOyS OF THE TfATfOXAL F.DVC.i- 

 TIOXAL ASSOCIATIOX. 



The committee on resolutions at the Boston 

 meeting of the National Educational Asso- 

 ciation, which consisted of Nicholas Murray 

 Butler, of New York, Chairman; Andrew 

 S. Draper, of Illinois; James M. Green, of 

 New Jersey; Bettie A. Button, of Ohio; H. 

 B. Frissell, of Virginia ; prepared the follow- 

 ing declaration, which was adopted by the 

 association. 



