232 



NATURE 



[January 3, 1901 



Artificial Rain. 



■ Af-ter' the magisterial words of Prof. Cleveland Abbe, as 

 reported in your issue of December 13, 1900, it requires some 

 courage to offer a possible instance of " artificial " rain. I was 

 near Bolton Abbey railway station on November 26 last. The 

 atmosphere was perfectly calm, and a thin white mist enveloped 

 the landscape. A number of land-blasting explosions took 

 place in some limestone quarries, perhaps a quarter of a mile 

 away. At a very short interval after these there occurred a 

 very little shower or sprinkling of rain, just sufficient to cause 

 me to put up my umbrella in preparation for more. The 

 extreme brevity of the shower, a:nd the peculiar conditions under 

 which it occurred, arrested my attention, and led me at once to 

 refer it to the explosions which had just taken place. 



Keighley, December 22, 1900. C. H. B. WooDD, 



In an article on artificial rain in your issue of December 13, 

 1900, Prof. Abbe alludes to " the popular belief that rain follows 

 great battles," which is now often — incorrectly, as the article 

 points out — explained by and used as an indication of some 

 effect produced on the clouds by the explosion of the gun- 

 powder. 



It is interesting in this connection to observe that the belief 

 about rain following battles was held many centuries before the 

 invention of gunpowder. Plutarch, in his life of Caius Marius, 

 writes: — "It is observed, indeed, that extraordinary rains 

 generally fall after great battles : whether it be that some 

 deity chooses to wash and purify the earth with water from 

 above ; or that the blood and corruption, by the moist and heavy 

 vapours they emit; thicken the air, which is generally liable to 

 be affected and altered by the smallest cause." (Langhorne's 

 translation). The inference is that the belief was the result of 

 a preconceived idea, and that the gunpowder explanation was 

 therefore wasted on a theory which was not grounded on obser- 

 vation at all. M. T. Tatham. 



Northcourt, Abingdon. 



PROGRESS IN METALLOGRAPHY. 



T^HE application of niicrographic analysis to the study 

 -*■ of alloys has given to the metallurgist a new and 

 important field of investigation, and every improvement 

 in the established methods is worthy of attention. Some 

 of the latest suggestions are made by M. Henri le 

 Chatelier in the Bulletin de la Society (T Encourao;enient 

 for September last, the most noteworthy being in con- 

 nection with the final stages of polishing. It is necessary 

 for this work that the polishing powders should be per- 

 fectly classified according to the dimensions of the par- 

 ticles. The method of sorting by means of levigation, 

 described by M. Osmond, is defective, owing to the fact 

 that the salts of lime in ordinary water cause coagulation 

 and rapid deposition of minute particles suspended in 

 water. Caustic lime and acids induce even more rapid 

 settling, a fact that has proved of great commercial im- 

 portance in the treatment of ore slimes by cyanide in 

 South Africa. 



To overcome this difficulty the powders are heated 

 with nitric acid, washed thoroughly, and allowed to settle 

 in distilled water containing 0*2 per cent, of ammonia. 

 When treating ten grams of powder in a litre flask, nine- 

 tenths of the liquid are siphoned off at the following 

 intervals of time : a quarter of an hour, one hour, four 

 hours, twenty-four hours, and eight days. The third 

 deposit is useful in polishing hard metals such as iron, 

 but the fifth and last deposit affords the best polishing 

 powder. Minute care is taken to avoid any admixture of 

 dust or dirt with these powders, which can now be bought 

 in Paris mixed into a paste with soap, and contained in 

 tin tubes such as are used for oil colours. 



A number of materials for the manufacture of these 

 powders have been tried. M. Le Chatelier finds that 

 alumina prepared by calcining ammonium alum is the best, 

 as far as speed of polishing is concerned ; but oxide of 

 chromium, obtained from the combustion of bichromate 



NO. 1627, VOL. 63] 



of ammonium, atiswers fairly well in the treatment of iron 

 and steel, and is better than alumina for soft metals such 

 as copper. Oxide of iron is far less advantageous than 

 these substances, its action being very slow. The soap 



Fig. I.— Crystals of AIqCu. 



preparations are applied in ths oidinary way to discs 

 of wood or metal covered with skin or cloth and 

 capable of being revolved £t high velocity, the whole 

 operation of polishing proper being carried through by 

 their aid in five minutes. 



I 



Fig. 2. — Compound near AlCu. 



For examining and photographing the polished and 

 etched specimens under the microscope, M, Le Chatelier 

 proposes the use of monochromatic light such as that 

 derived from an electric arc in mercury vapour, with 



