September 22, 1904] 



NATURE 



523 



was charged with a solution of potassium bichromate, 

 another with copper ammonium sulphate solution, and the 

 third with pure water, and all were exposed to sunlight for 

 four hours. The deal in the red light gave only a faint 

 picture, that in the blue light a dark picture, and that with 

 the pure water was only a slightly darker picture. Resin, 

 guaiacum, copal varnish, w"hite oil paint and resin sized 

 paper all acted in the same way and gave similar results. 



The light from an arc lamp when passed through a red 

 glass and allowed to fall on a wood section for one and a 

 half hours produced no effect, but when the same light was 

 passed through a blue glass and fell on a similar wood 

 section for only one hour it produced a dark picture. With 

 liquids this same increase of activity by the action of blue 

 light is produced. Turpentine, which has been exposed to 

 blue light, is more active than when in its ordinary 

 condition. 



THE DENSITY OF MTKOi'S OXIDE.' 

 f N the Proceedings, vol. l.Kxii. p. 204, 1897," I have given 

 particulars of weighings of nitrous o.xide purified by 

 two distinct methods. In the first procedure, solution in 

 water was employed as a means of separating less soluble 

 impurities, and the result was 3 6356 grams. In the second 

 method a process of fractional distillation was employed. 

 Gas drawn from the liquid so prepared gave 3 6362. These 

 numbers may be taken to represent the corrected weight of 

 the gas which fills the globe at 0° C. and at the pressure of 

 the gauge (at 15°), and they correspond to 2 6276 for oxygen. 



Inasmuch as nitrous oxide is heavier than the impurities 

 likely to be contained in it, the second number was the 

 more probable. But as I thought that the first method 

 should also have given a good result, I contented myself 

 with the mean of the two methods, viz. 3 6359, from which 

 1 calculated that, referred to air (free from H,0 and CO,) as 

 unity, the density of nitrous oxide was i 52951. 



The corresponding density found by M. Leduc is 1-5301, 

 appreciably higher than mine; and M. Leduc argues that 

 the gas weighed by me must still have contained one or two 

 thousandths of nitrogen.^ According to him the weight of 

 the gas contained in my globe should be 3 6374, or 15 milli- 

 grams above the mean of the two methods. 



Wishing, if possible, to resolve the question thus raised, 

 I have lately resumed these researches, purifving the nitrous 

 oxide with the aid of liquid air kindly placed at my disposal 

 by Sir J. Dewar, but I have not succeeded in raising the 

 weight of my gas by more than a fraction of the discrepancy 

 (1-5 milligrams). I have experimented with gas carefully 

 prepared in the laboratory from nitrate of ammonia, but as 

 most of the work related to material specially supplied in an 

 iron bottle I will limit myself to it. 



There are two ways in which the gas may be drawn from 

 the supply. When the valve is upwards, the supply comes 

 from the vapourous portion within the bottle, but w-hen the 

 valve is downwards, from the liquid portion. The latter is 

 the more free from relatively volatile impurities, and accord- 

 ingly gives the higher w'eight, and the difference between 

 the two affords an indication of the amount of impurity 

 present. After treatment with caustic alkali and sulphuric 

 acid, the gas is conducted through a tap, w-hich is closed 

 when it is desired to make a vacuum over the frozen mass, 

 and thence over phosphoric anhydride to the globe. For the 

 details of apparatus, &:c., reference must be made to former 

 papers. 



The first experiment on July 13 was upon gas from the 

 top of the bottle as supplied, and without treatment by liquid 

 air, with the view of finding out the worst. The weight 

 was 3 6015, about 35 milligrams too light. The stock of 

 material was then purified, much as in i8g6. For this 

 purpose the bottle was cooled in ice and salt * and allowed 

 during about one hour to blow oil half its contents, being 

 subjected to violent shaking at frequent intervals. Sub- 

 sequently three weighings were carried out with gas drawn 

 from the bottom, but without treatment by liquid air. The 



1 By Lord Raylcieh. O.M.. F.K.S. Abridged from a paper received at 

 Ihe Royal Society on September i. 



- Or ** Scientific Papers," vol. iv. p. 350. 



■■! " Recherches sur les Gaz " (Paris, 1898.) 



■* The lower the temperature below the critical point, the more effective 

 is this procedure likely to be. 



NO. I 82 I, VOL. 70] 



results stand :— July 18, 3()368 ; July 20, 36360; July 25, 

 3 6362 ; mean, 3-63t>33. 



Next followed experiments in which gas, still drawn from 

 the bottoiTi of the bottle, was further purified bv condensation 

 with liquid air. On one occasion (August 7) the condensed 

 gas was allowed to liquefy, for which purpose the pressure 

 must rise to not far short of atmospheric, and to blow off 

 part of its contents: — August i, 36363; August 3, 3-6367; 

 August 7, 36366; mean, 3-63653. 



The treatment with liquid air raised the weight bv only 

 o 2 milligram, but the improvement is probably real. " That 

 the stock in the bottle still contained appreciable impurity 

 IS iiidicated by a weighing on August 13, in which without 

 liquid air the gas was drawn from the top of the bottle. 

 There appeared, August 13, 3-6354, about i milligram short 

 of the proper weight. 



It will be seen that the result without liquid air is almost 

 identical with that found by the same method in 1S96, and 

 that the further purification bv means of liquid air raises the 

 weig-ht only to 36365. I find it, difficult to believe that so 

 purified the gas still contains appreciable quantities of 

 nitrogen. 



The corresponding weight of air being 2-3772,' we find 

 that, referred to air as unity, the density of nitrous oxide 

 '^ 3 6365/2-3772 = i-52q7- Again, if oxygen be taken as 16, 

 the density of nitrous oxide will be 3-6365x16/2-6276 = 

 22-143. 



The excess above 22 is doubtless principally due to the 

 departure of nitrous oxide from Boyle's law between atmo- 

 spheric pressure and a condition of great rarefaction. I 

 hope shortly to be in a position to apply the connection which 

 will allow us to infer what is the ratio of molecular weights 

 according to Avogadro's rule. 



UNIVERSITY AND EDUCATIONAL 

 INTELLIGENCE. 



Mr. Ernest Shearer, Kirkwall, has been appointed 

 lecturer on agriculture at the Pusa Imperial College, Bengal. 

 This model agricultural college for all India, with a farm 

 of 1300 acres attached, is one of the admirable developments 

 resulting from the appointment two or three years ago of 

 Mr. James Mollison as Inspector-General of Agriculture in 

 India. Mr. Alexander Sangster, Montrose, has been 

 appointed junior assistant with the .Aboukir Land Reclam- 

 ation Co., near .Alexandria, Egypt, and Mr. John C. Leslie 

 assistant conservator of forests in southern Nigeria. 



The approach of the new sessions at polytechnics and 

 similar institutes is heralded by the appearance of calendars 

 and prospectuses, several of which have been received 

 within the past few days. At the Birkbeck College, 

 Chancery Lane, the session will commence on Monday, 

 October 3, when an inaugural address will be delivered by 

 Dr. J. E. Mackenzie on " The Influence of Pure Science on 

 Progress." The class-rooms and laboratories of the college 

 will afterwards be open to inspection, and demonstrations 

 will be given. A course in science with practical work has 

 been organised to give complete preparation in metallurgy 

 and mining for those qualifying for the mining profession. 

 It is satisfactory to know that within the last few vears 

 valuable reference libraries have been provided for the 

 separate departments of science ; these have been aided by 

 grants from the County Council. His Majesty's Treasury 

 recently presented to the college forty-nine volumes of the 

 scientific results of the Challenger Expedition. 



Three prospectuses have been received from the South- 

 western Polytechnic, referring respectively to the day college 

 for men and women, day school for boys and girls, and 

 evening classes. The principal of the polytechnic is Mr. 

 -S. Skinner. The courses at the day college are arranged 

 to occupy three years. On entering the student has to state 

 whether he wishes to be trained as a mechanical, civil, or 

 electrical engineer, or as a consulting or industrial chemist. 

 In any of these cases he will find mapped out for him a 

 complete course of study, involving laboratory instruction, 

 tutorial work, attendance at lectures, exercises in mathe- 

 matics, geometrical, mechanical and architectural drawing, 

 and instruction in the workshops. 



1 Roy. S.yz. Prjc, vo". liii. p. 131, .333; 'Scientific Paptrs," vol 

 p. 47- 



