l ; 2<S Notices respecting New Books. 



same deflection produced by bringing up the N pole of the 

 magnet WW. The magnets are made of equal pole-strength 

 and tested in the usual manner; the distance between them is 

 taken in centimetres, the weight Q being in parts of the 

 gramme. In the formula which shows the relationship be- 

 tween magnetic forces, /=- — #— (where /= force, mm' pole- 

 strengths, d distance between them), if m = m' as in this instru- 

 ment, then /= -» ; and if / be replaced by Qg (i. e. the 

 numerical value of the weight x gravitation), the formula 

 becomes Qg= -p from which m is known in absolute measure. 



With the pole-strength thus obtained the strength of other 

 magnets and solenoids can be readily compared. In order to 

 secure great strength, the frame which carries the torsion-wire 

 is a gun-metal casting, having a rib at the back. 

 Taunton, July 27, 1882. 



XXVII. Notices respecting New Boohs. 



A Treatise on the Distillation of Coal-Tar and Ammoniacal Liquor. 

 By Geobge Lunge, Ph.D., F.C.S., Professor of Technical Che- 

 mistry in the Federal Polytechnic School, Zurich. Loudon : Van 

 Voorst, 1882. 

 T~\B. LU^GKE is so well known, not only from his published 

 *-* investigations, but also from his ' Treatise on the Manufac- 

 ture of Sulphuric Acid and Alkali,' that we were desirous of ascer- 

 taining how he would handle the subjects of coal-tar and ammoniacal 

 liquor. A careful examination of his book enables us to pronounce 

 most favourably upon it ; and we regard it as a very valuable 

 contribution to the literature of coal-tar and its derivatives. 



Dr. Lunge commences his work by a preliminary chapter of 25 

 pages entitled " The Origin of Coal-tar," in which he discusses the 

 differences between the tars derived from peat, browncoal, bitu- 

 minous coal, and real coal, and also the effect of the temperature 

 at which the coal is distilled upon the resulting products. We 

 quite coincide with the author that the system well-nigh universally 

 adopted, of squeezing the maximum quantity of gas out of the coal 

 and letting the tar come out as it may, is a bad system. We may, 

 perhaps, be hardly prepared to accept the dictum recently uttered 

 at the annual meeting of gas-managers by a well known American, 

 that, before long, gas will become the " residual " and coal-tar the 

 principal object in the carbonization of coal ; nevertheless it is, we 

 think, indisputable that coal-tar is a necessity of the age, and will 

 become rapidly of greater and greater value. 



In Chapter II. a table is given of the constituents of coal-tar. 



