April 20, 1876] 



NATURE 



489 



may best be exhibited so as to satisfy all the purposes 

 for which they are intended. M. Felix Plateau, at a 

 former meeting, proposed to subsiitute yellow for colour- 

 less glass in lighting rooms containing entomological col- 

 lections. In the discussion which followed it was sug- 

 gested that experiments should be made by submitting 

 insects to the intluence of glasses of various colours. M. 

 Capronnier was entrusted with carrying out these experi- 

 ments, and the paper referred to contains his report. 



Everyone knows that among the Lepidoptera it is the 

 preen and carmine colours which are most rapidly 

 destroyed by daylight. M. Capronnier wished to obtain 

 insects of the year's hatching, but could only obtain 

 sufficient quantities of Euchelia JacobcBoe, L. The inferior 

 wings of this insect are of a deep carmine, uniform in 

 tone, an important point in the experiments. 



The principal colours of the solar spectrum are the 

 yellow, the red, the blue. M. Capronnier rejected the 

 red as giving a tint too dark, and added the mixed 

 colours, violet and green. He had thus four tints chosen 

 with the same degree of tone, and of a moderate shade — 

 yellow, violet, green, and blue, besides a colourless glass. 

 He made five small square boxes of "oS centimetres 

 square and one centimetre in depth ; the whole surface 

 was covered with one of the above-mentioned glasses. 



Each wing was fixed in the middle of the box and 

 floated in a bath of very bright light, but protected from 

 the rays of the sun. Each of the wings was partly 

 covered by a band of black paper, and their position was 

 so arranged as to leave exposed successively each of the 

 parts during a period of fifteen, thirty, and ninety days. 

 The following are the results : — 



Colourless glass. — After fifteen days of exposure the 

 carmine tint was visibly attacked. After thirty days the 

 alteration was more sensible, and after ninety days the 

 work of destruction had rapidly advanced, and the car- 

 mine had passed into a yellowish tint. 



Blue. — With this tint the same alterations took place as 

 in the case of colourless glass. 



Green. — This colour preserved the carmine during the 

 first fifteen days ; a change was indicated on the 

 thirtieth day, and on the nintieth the alteration was 

 marked. 



Yellow. — During the ninety days the ytllow alone left 

 the carmine colour almost intact. M. Capronnier says 

 abnost, for a slight alteration in the tint could be noticed 

 at the end of the ninety days. This last observation 

 proves that there is no absolute preservative, and that 

 collections must be kept in darkness, under penalty of 

 seeing them seriously changed at the end of a given 

 time. 



Nevertheless, it is evident from the above that the 

 yellow is the best preservative against alterations in the 

 colours of insects. M. Capronnier consequently con- 

 cludes that a yellowish colour should be preferred and 

 combined in every arrangement of an entomological 

 room. I^Ioreover the cloths that cover the show-cases 

 ought to be yellow rather than green, and what is im- 

 portant and indispensable, the window-blinds ought to be j 

 absolutely yellow. 



RADIOMETERS'- 

 TOURING the discussion which followed the reading of 

 -*-^ Prof. Reynolds's and Dr. Schuster's papers at the j 

 last meeting of the Royal Society I mentioned an experi- j 

 ment bearing on the observations of Dr. Schuster. I ; 

 have since tried this in a form ; and as the results are 

 very decided and appear calculated to throw light on 

 many disputed points in the theory of these obscure 

 actions, I venture to bring a description of the experiment, | 

 and to show the apparatus at work, before the Society. 1 

 I made use of a radiometer described in a paper com- 



' " On the Movement of the Glass Case of a Radiometer." By William | 

 Crookes, F.R.S., &c. Read at the Royal Society. I 



. municated to the Society in January last. I quote the 

 description from paragraph 184. "A large radiometer 

 in a 4-inch bulb was made with ten arms, eight of them 

 being of brass, and the other two being a long watch-^ring 

 magnet. The discs were of pith, blackened on one side. 

 The power of the earth on the magnet is too great to 

 allow the arms to be set in rotation unless a candle is 

 brought near, but once started it will continue to revolve 

 with the light some distance off'." 



This radiometer was floated in a vessel of water and 

 four candles were placed round it, so as to set the 

 arms in rotation. A mark was put on the glass envelope 

 so as to enable a slight movement of rotation to be seen. 

 The envelope turned very slowly a few degrees in one 

 direction, then stopped and turned a few degrees the 

 opposite way ; finally it took up a uniform but excessively 

 slow movement in the direction of the arms, but so slow 

 that more than an hour would be occupied in one revolu- 

 tion. 



A powerful magnet was now brought near the moving 

 arms. They immediately stopped, and at the same 

 time the glass envelope commenced to revolve in the 

 opposite direction to that in which the arms had been 

 revolving. The movement kept up as long as the 

 candles were burning, and the speed was one revolution 

 in two minutes. 



The magnet was removed, the arms obeyed the force 

 of radiation from the candles, and revolved rapidly, whilst 

 the glass envelope quickly came to rest and then rotated 

 very slowly the same way as the arms went. 



The candles were blown out ; and as soon as the whole 

 instrument had come to rest, a bar-magnet was moved 

 alternately from one side to the other of the radiometer, 

 so as to cause the vanes to rotate as if they had been 

 under the influence of a candle. The glass envelope 

 moved with some rapidity (about one revolution in three 

 minutes) in the direction the arms were moving. On re- 

 versing the direction of movement of the arms the glass 

 envelope changed direction also. 



These experiments show that the internal friction, either 

 of the steel point on the glass socket, of the vanes against 

 the residual air, or of both these causes combined, is 

 considerable. Moving the vanes rcund by the exterior 

 magnet carries the whole envelope round in opposition 

 to the friction of the water against the glass. 



As there is much discussion at present respecting the 

 cause of these movements, and as some misunderstand- 

 ing seems to prevail as to my own views on the theory of 

 the repulsion resulfing from radiation, I wish to take this 

 opportunity of removing the impression that I hold opinions 

 which are in antagonism to some strongly urged expla- 

 nations of these actions. I have on five or six occasions 

 specially stated that I wish to keep free from theories. 

 During my four years' work on this subject I have 

 accumulated a large fund of experimental observations, 

 and these often enable me to see difficulties which could 

 not be expected to occur to an investigator who has had 

 but a limited experience with the working of one or two 

 instruments. 



COMPRESSED AIR LOCOMOTIVE USED IN 

 THE [ST. GOTHARD TUNNEL WORKS^ 



THE boring of a tunnel of any importance presents 

 difficulties of various kinds, among which may be 

 mentioned the clearing away of the rubbish arising from 

 the excavation of the gallery, whenever that reaches any 

 considerable length, and the work is carried on with 

 activity. Such were the conditions under which the 

 boring of the Mont Cenis tunnel was carried on, and 

 M. Fabre, the able contractor, has met with similar diffi- 

 culties in the boring of the St. Gothard tunnel, now being 

 carried out. 



* From an article in La Nature, by M. C. M. Gariel. 



