38 REPORT— 1872. 



Since the publication of the results of the author's first experiments, made in 

 1863, there has been quite a change in the original colour of some of the -n-indow 

 glass made in Europe. Tlie author imderstauds that many of the manufactui-era 

 have given up the use of oxide of manganese, or reduced the quantity employed. 

 The result is, tliat the brownish-yellow coloured glass, which used to change to a 

 pm-ple hue in a year or less, is now replaced by a light bluish green, which shows 

 little or no change after years of exposure. It will be a practical result of the 

 inquiry suggested above, if colourless glass of all kinds shall be made which shall 

 not change in colour by sunlight exposure, and but slightly in shade. Especially 

 is it important to photogTaphers, in any operations requiring all the light which 

 they can obtain, not to have glass in their skylights which, after a few months or 

 years of exposure, shall be robbed of a great proportion of its power to transmit 

 the chemical or actinic rays, by a change to a j'ellow or purple hue, which, in 

 time, might cut off almost as much actinic effect as if it were ground or covered 

 with enamel on one of its surfaces. The author made some photographic experi- 

 ments to show this deteriorating effect, by exposing sensitive paper under glasses 

 of original colour, and those of the same kind clianged by sunlight, and witnessing 

 the very perceptibly different shades of darkening produced. 



This action of sunlight must not be confounded with rust or stain occasioned by 

 exposure to atmospheric influences, which occasions sometimes a roughening and 

 sometimes an iridescence upon the surface ; while sunlight action, which has no 

 disintegrating effect on the outside, extends throughout the body of the glass. 



On the Spectrum of Hydrogen. By Aethtje ScnrsTEK. 



Hydi'Ogen is one of the gases said to exhibit more than one spectrum. Under a 

 pressure greater than about o millimetres it is said to show a spectrum of shaded 

 bands. The spectrum of hydrogen which is seen in the heavenly bodies appears 

 under a pressure from 5 millimetres down to the lowest pressure which can be ob- 

 tained by Sprengel's pump, where a new spectrum of lines suddenly appears. 

 Plucker, who discovered the band-spectrum of hydrogen, was first of the opinion 

 that it was due to the last traces of air. Finding, however,Qthat its bands did not 

 coincide with the bands of air, he attributed it to hydrogen. Angstrom has recently 

 given his reasons against this supposition, and believes it^, to be due to acetylene. 

 My own experiments have led to the confirmation of Angstrom's opinion. Gene- 

 rally two distinct causes may introduce a hj-drocarbon into the vacuum-tube : — 



1. The gas passing through india-rubber tubes will carry with it small pieces of 

 india-rubber. 



2. All the vacuum-tubes are more or less greasy. 



These two causes I consider sufiicient to produce all the effects obsened 

 by Pliicker. Wiillner, however, found this spectrum so well developed that 

 we must look in his experiments for a more constant soiu-ce of error. This, 

 I believe, is found in the greased stopcocks which he used to shut his vacuum- 

 tubes. Examining the spectrum of oxygen, he discovered two new spectra 

 •which he found later to be due to carbon-compounds introduced into his vacuum- 

 tubes by the grease of the stopcocks. The quantity of solid matter carried 

 away by a current of air passing through an india-rubber tubing is not so 

 small as might at first sight appear. Tyndall, in his experiments on actinic 

 clouds, mentions the effect produced by an india-rubber joint through which the 

 gas, subjected to examination, had passed. In order to eliminate the eflect of the 

 tubings, a drop of water was introduced into the vacuum-tube, which was boiled 

 after the vacuum had been made. When all the air was expelled the spark was 

 allowed to pass. It was now found that the band-spectrum varied much with 

 the different tubes. „ Those which had been well cleaned before being used showed it 

 only very feebly. Angstrom's supposition that this spectrimi is due to acetylene is 

 therefore very plausible. 



I obtained the spectrum of ammonia by the following arrangement : — A few drops 

 of a strong solution of ammonia in water were introduced into the vacuum-tube, 

 and the induction-current was allowed to pass while the pump was being worked. 



