460 Notices respecting New Boohs, 



Thus when the disturbance is given and the plane-wave com- 

 ponents are determined so as to satisfy the initial conditions 

 throughout all space, then this same plane wave system will; 

 continue to represent the subsequent disturbance in all its 

 stages, and the waves will be propagated as plane waves. 



As Dr. Stoney contrasts the method adopted in my com- 

 munication with that employed by himself elsewhere, I feel 

 bound before concluding to take this opportunity of pro- 

 testing against the method adopted by Dr. Stoney in his 

 proof of the theorem on p. 276. I object to the ease and 

 freedom with which he rides off to infinity on a spherical 

 wave and comes back on a plane wave. One does not feel 

 quite sure as to what has happened in the meantime. Why 

 go to infinity in order to find out what is going on about 

 home ? Why deal with a very long cylinder of .finite width 

 rather than a very narrow one of moderate length ? If it is 

 true that a curved wave may be replaced by its tangent planes, 

 considered as infinite plane waves, this should be demonstrated 

 about home rather than at infinity. A spherical sector of 

 moderate area certainly approximates to the corresponding 

 area in the tangent plane as the radius of the sphere increases. 

 But this sector travels out as part of a complete spherical wave, 

 while, when reversed, it returns as a segment of a wave. How 

 does it return ? Is it supposed to be geometrically reversed, 

 so as to focus at the original centre, or does it diffuse through- 

 out space by diffraction during the whole time of its return ? 

 Is it evident by any method that this sector, when reversed, 

 will produce the same effect at the original centre as the 

 whole tangent plane wave would ? 



Such are some of the difficulties which I recognize in the 

 method, and these are real and great difficulties to those who 

 are less deeply versed than Dr. Stoney in the philosophical 

 aspect of the subject. 



I am, Gentlemen, 

 Bardowie, Faithfully yours, 



Thomas Preston. 



LX. Notices respecting Neio Boohs, 



The Phase Rale. By Wildek D. Bancroft. Ithaca, New York. 

 The Journal of Physical Chemistry, 1897. 



T\^E have been chiefly indebted to Dutch and German workers 

 ' * for the great advances made in our knowledge of physical 

 chemistry during the pyst few years. Now, however, the school 

 of physical chemists in America has shown its existence and 

 activity by the publication of a monthly journal of which Professor 



