August i i, 1923] 



NA TURE 



22 



The thermal and chemical efficiencies of manu- 

 facture of different grades of gas by various processes 

 have been the subject of three reports by a Joint 

 Committee of the University of Leeds and the Institu- 

 tion of Gas Engineers. The first Report dealt with 

 the process of steaming the charge in continuous 

 vertical retorts, and the results showed that the 

 thermal efficiency of gas production increased from 

 54-5 per cent, without steam to a maximum of 62-1 

 per cent, with moderate steaming, and at the same 

 time increased yields of tar and ammonia were 

 obtained. These results were later confirmed by 

 work carried out about the same time b5M;he Fuel 

 Research Board. The second Report showed that 

 the efficiency of production of blue water gas as 

 ordinarily practised in a plant without waste heat 

 boilers, taking into account the steam required for the 

 operation of the plant, averaged 46 per cent. In the 

 third Report on the subject (contained in the Com- 

 mittee's Seventh Report, a copy of which has just 

 been received, presented to the Institution of Gas 

 Engineers in June 1922), the Committee shows that 

 the percentage thermal efficiency of production of 

 carburetted water gas of calorific value about 485 

 B.Th.U. per cubic foot, taking into account all steam 

 required, was increased from 59"5 per cent to 68 per 



cent., by the use of waste heat boilers employed for 

 steam raising by means of waste heat in the flue 

 gases. The efficiency of production of blue water 

 gas was 53 per cent, and of the production of gas from 

 oil for carburetting 90 per cent. The percentage 

 thermal efficiency of the waste heat boilers averaged 

 only about 46 per cent. 



The problem foremost in the mind of the gas 

 industry to-day is the production and distribution 

 of the Therm at the cheapest price. The maximum 

 conservation of coal within the industry will be 

 achieved when that problem has been settled. While 

 the Fuel Research Board could not, from the nature 

 of the problem, specify any one grade of towns' gas 

 as being under all conditions most suitable for 

 production and distribution, its recommendations, 

 embodied in the Gas Regulation Act, 1920, do, for 

 the first time in the history of the industry, enable the 

 relative efficiencies of gas production by various 

 processes and in different parts of the kingdom to be 

 compared on a scientific basis. The work of the 

 Committee to which reference is here made is evidence 

 of the quickened interest on the part of the gas 

 industry in these matters and an earnest of higher 

 efficiencies yet to be realised, and a cheaper Therm 

 still to be distributed. J. S. G. T. 



Optical Works of Messrs. Adam Hilger, Ltd. 



''"PHE show-rooms of Messrs. Adam Hilger, Ltd., 75A 

 '- Camden Road, London, N.W.i, contain a very 

 interesting exhibition of optical instruments, to the 

 inspection of which visitors are cordially invited. A 

 short account of some of the devices and operations 

 seen during a recent visit to the works may be of 

 interest to readers of Nature. 



In a room devoted to the grinding and polishing of 

 lenses and mirrors, a recently silvered mirror was being 

 coated with a thin varnish to preserve the surface of 

 the film which was not in contact with the glass, and 

 was to be used to reflect light in an optical instrument 

 in the same way that a silvered mirror is used in an 

 astronomical telescope. The mirror was circular and 

 about 4 inches in diameter, cathodically silvered. 

 It was mounted by soft wax on a wooden mandril 

 which revolved on a vertical shaft at some thousand 

 revolutions per minute with its silvered surface upper- 

 most. Dust was brushed from the surface by means 

 of a fine camel's hair brush, and then a weak solution 

 of celluloid in amyl acetate was poured upon it and 

 left to dry, while the mirror was rapidly revolving. 

 This left a thin film of celluloid on the mirror, which 

 preserves its brightness. Films which are thick 

 compared with a wave-length of light protect the 

 silvered surface almost indefinitely, but these do 

 not allow of the highest definition. On the other 

 hand, films which are thin compared with a wave- 

 length of light do not preserve the silver so well, 

 but do not, however, in any way adversely affect the 

 optical performance of the mirror. Films of inter- 

 mediate thickness would tend to produce colours on 

 the principle of Newton's rings. 



Several prisms of rock salt were seen in process of 

 manufacture ; these cannot be ground with water as 

 in the case of glass, owing to its dissolving action on 

 the substance, so paraffin is used instead, and the 

 accuracy of the rough grinding is tested by steel sets 

 of 60° angle. All finished optical surfaces are, of 

 course, tested by interference methods, the source of 

 light being the mercury vapour arc. A Lummer 

 plate was being tested by this means. Newton's 

 rings were used, and they were plainly visible in spite 

 of the thickness of the plate. 



NO. 2806, VOL. 112] 



In another room the thickness of a piece of plain 

 parallel quartz some i J in. x ij in. x \ in. was being 

 measured on a Michelson interferometer. The half- 

 coating of silver had been removed from one of the 

 mirrors of the instrument, and the specimen was then 

 " contacted on " to this mirror so as to cover one 

 half of it. The whole was then half silvered, and 

 the distance between the two surfaces was measured 

 in air. What appeared to be a slight scratch in the 

 centre of the specimen was in reality a slit in the glass, 

 of width only 16 wave-lengths of light (X5461). 

 This slit was made in the manner illustrated in the 

 accompanying diagram (Fig. i). It will be seen that 

 the quartz plate was in reality built up of four pieces, 

 all optically finished with extreme 

 accuracy. Starting with i, 2 and 



3 were contacted on and heated 

 sufficiently to make these three 

 join up into one piece, but, of 

 course, not too much, or the 

 optical perfection of the surfaces 

 would be spoilt. The protruding 

 edges of 2 and 3 where they meet 



4 were then ground and polished 

 so that they extended beyond i a 



distance equal only to 16 wave-lengths. This distance 

 was measured with the Michelson interferometer, and 

 then 4 was contacted on and the heating process 

 repeated. 



In the workshops an accurate screw was being cut 

 similar to that which is used in the Fabry and Perot 

 interferometer. This was done on an automatic 

 electrically controlled lathe. Whenever the cutting 

 tool reached the end of its stroke, electrical contacts 

 were made, which moved the tool away and brought 

 it back to the beginning of the next cut. The screw, 

 when turned, is rotated from end to end tlirough a 

 long split nut, driven by an electric motor with an 

 automatic reversing gear. The nut contains a 

 thermometer, the temperature of which is read from 

 time to time to avoid over-heating, and this is appar- 

 ently sufficient for the purpose, though one might have 

 thought that an oil-bath would have kept the 

 temperature more constant. Great care has to be 



Fig. 



