Chemistry and Physics. 69 



Figuren im Text. Leipzig, 1897. (Wilhelm Engelman.) 3 

 marks. — The brochure before us contains the substance ot the 

 author's lectures given in Amsterdam in 1894-5 and in Berlin in 

 1896, upon the formation and decomposition of double salts, being 

 a resume of the investigations made by the author and his students 

 upon this subject. In putting it into print the lecture form has 

 been changed and the matter has been arranged under three sub- 

 divisions. In the first, which is theoretical and general, the 

 action of a sparingly soluble double salt formed by the union of 

 two binary salts, is considered, with reference to the author's 

 theory of dilute solutions and of electrolytic dissociation. In the 

 second, the experimental methods made use of in the investiga- 

 tions are described, these methods being original and most sug- 

 gestive. In the third, which is special, the behavior of certain 

 salts, such as potassium-cupric chloride, CuCl 2 . (KC1) 2 . (H 2 0) 2 , 

 schonite, MgK 2 (S0 4 ) 2 . (H 2 0) 6 , the racemates of ammonium and 

 sodium and of potassium and sodium and the dextro- and laevo- 

 rotatory Rochelle salts is given at length and shown to be in 

 accord with theory. It is evident therefore that a distinct pro- 

 gress has been made by Professor Van't Hofi over the results 

 given in his " Chemical Dynamics." In the latter book the ques- 

 tion of temperature was the one mainly considered, while in the 

 present work all the other conditions upon which the existence 

 of double salts depends are studied minutely. The book. is a 

 most valuable addition to the literature of physical chemistry. 



G. F. B. 



1. The Limits of Audition j by Lord Rayleigh. (Abstract.) 

 — In order to be audible, sounds must be restricted to a certain 

 range of pitch. Thus a sound from a hydrogen flame vibrating 

 in a large resonator was inaudible, as being too low in pitch. On 

 the other side, a bird-call, giving about 20,000 vibrations per 

 second, was inaudible, although a sensitive flame readily gave 

 evidence of the Vibrations and permitted the wave-length to be 

 measured. Near the limit of hearing the ear is very rapidly 

 fatigued; a sound, in the first instance loud enough to be disa- 

 greeable, disappearing after a few seconds. A momentary inter- 

 mission, due, for example, to a rapid passage of the hand past 

 the ear, again allows the sound to be heard. 



The magnitude of vibration necessary for audition at a favora- 

 ble pitch is an important subject for investigation. The earliest 

 estimate is that of Boltzmann. An easy road to a superior limit 

 is to find the amount of energy required to blow a whistle and 

 the distance to which the sound can be heard (e.g. one-half a mile). 

 Experiments upon this plan gave for the amplitude 8X10 -8 cm., 

 a distance which would need to be multiplied 100 times in order 

 to make it visible in any possible microscope. Better results 

 may be obtained by using a vibrating fork as a source of sound. 

 The energy resident in the fork at any time may be deduced 

 from the amplitude as observed under a microscope. From 

 this the rate at which energy is emitted follows when we know 



