1 86 



NATURE 



[June 24, 1897 



PROFESSOR MENDEL^EFF ON THE HEAT 



OF COMBUSTION} 

 T^ULONG'S formula, which gives the heat of combustion of 

 ■*-^ different solid and liquid fuels, as depending upon their 



composition. 



known, ^ = 8i c + 345 {^' - ^ )> 



c, h and 



representing the percentages of carbon, hydrogen, and oxygen 

 in the fuel. 



If a general expression, = Ac + BA - Co, be taken, the 

 numerical value of the coefficient A = 81 must be maintained, 

 because it corresponds to pure charcoal, and all known data 

 (from 8140 to 8060) prove that the figure 81 must really be taken 

 for each per cent, unit of carbon in the fuel (the accuracy of the 

 measurements being within the limits of from i to 2 per cent, of 

 the total heat of combustion). For hydrogen, however, the co- 

 efficient B = 345 cannot be maintained, because it has been 

 obtained out of data relative to the burning of gaseous hydrogen, 

 while in usual solid or liquid fuel the elasticity of the gas is 

 lost ; its hydrogen must be considered as if it were liquefied, 

 and consequently B must not be, according to what is known, 

 more than 300 (admitting, as is usually done, that the water 

 obtained during combustion is in a liquid state). - 



In order to find the true coefficients suitable for practical pur- 

 poses, Mendeleeff took the figure (p — 4190, which is quite 

 correct (within I per cent.) for pure cellulose, as also the 

 average from 79 most complete measurements for fat coals (by 

 Maler, Alexeyeff, Damski, Diakonoff, Miklaschewski, Schwan- 

 hfifer, and Bunge), and the average for naphtha fuel, and he has 

 found : 



<p—%\c'+ Tpoh - 26 {0 - s), 

 which formula represents,^ with an accuracy of from i to 2 

 per cent. , the heat of combustion of pure charcoal, coke, coals, 

 lignites, wood, cellulose, and naphtha fuels ; of course, it applies 

 to the best determinations only, especially to those which were 

 made in a calorimetric bomb, where the error is less than i to 

 2 per cent.* 



This formula is an approximate empirical expression of facts ; 

 but it corresponds at the same time to the numerical value of 

 the coefficient B for hydrogen, which could be expected from 

 theoretical considerations.'* 



O^ 



THE SCIENTIFIC REQUIREMENTS OF 



COLOUR PHOTOGRAPHY. 

 |N Tuesday evening, June i, in the Examination Schools, 

 Oxford, Captain W. de W. Abney, C.B., F.R.S., gave 

 the sixth Robert Boyle Lecture before the Oxford University 

 Junior Scientific Club. The President, Mr. R. A. Buddicom, 

 was in the chair, and about 800 members and their guests were 

 present. 



The subject chosen by Captain Abney was " The Scientific 

 Requirements of Colour Photography." The following is an 

 abstract of the lecture : — 



Colour photography and photography in natural colours are 

 two distinct methods of arriving at the same end, namely, the 

 production of a picture of objects, coloured as they naturally 

 appear to the eye. Both have been accomplished and depend 

 on the application of science, but in the case of the former 

 additional knowledge is requisite of the mode of action of the 

 retina and of theories of colour vision. 



In colour photography the theory of colour vision usually 

 adopted is the Young-Helmholtz three-colour theory, in which 

 red, green and blue are selected as primary colours, and not the 

 red, yellow, blue of the artist. Captain Abney pointed out the 

 difference between colour and colour sensation, and placed his 

 colour sensation curves before the audience. These curves 

 enable particular coloured screens to be selected, .so that if 



1 Translated from the Journal of the Russian Chemical and Physical 

 Society, vol. -xxix. fasc. 2, 1897, pp. 144. (Minutes of meeting of February 

 18, 1897.) 



-_ Maler has also adopted that coefficient, taking C = 30. 



^ The percentage of sulphur was not determined in each measurement, 

 and consequently the coefficient + 26 is determined only approximately. 



4 If the water which is formed during combustion is represented, as it is in 

 reality, in the shape of steam, then 600 a/f must evidently be deducted from 

 </> ; ag representing the weight of water obtained from the combustion of one 

 unit of fuel. 



5 In those cases where different values of <p were received for the same 

 composition of coal, the discrepancies could be explained by errors of measure- 

 ments ; there was no foundation to suspect isomerism. A good deal of the 

 now prevaihng incertitude is also due to the incomplete data relative to the 

 r of combustible sulphur. 



NO. 1443. VOL. 56] 



transparencies from three photographs of the same object, taken 

 one through an orange screen, one through a green, and one 

 through a blue, be each illuminated by its own peculiar coloured 

 light, and the three images be superimposed, the effect is to re- 

 produce a picture of the object in its original colours. The 

 colours of the screens used for taking the negatives must not be 

 such as to allow only monochromatic light to pass. Thus the 

 red screen must allow some orange, the green some yellow, and 

 the blue some green, so that the lights through the three screens 

 overlap somewhat. 



The viewing screens, on the contrary, should be as nearly 

 monochromatic as possible. By these means Mr. Ives has, in 

 his chromoscope, been able to present to view photographs of 

 natural objects in the colours in which they appear to the eye. 



The next process described was that of Dr. Joly, of Dublin, 

 who, basing his work on the same theory of colour vision as Mr. 

 Ives, reproduces in colour by means of a single negative. This 

 method is essentially founded on what may be called a happy 

 imperfection of the eye. The human eye is incapable of 

 separating points which lie very close to one another. In an 

 engraving, the black lines, close together on a white surface, 

 blend with the white surface to form shades of grey. Dr. Joly's 

 method is to rule lines 1/200 inch broad on a transparent screen, 

 touching one another and being coloured alternately red, green 

 and blue. The lines are of such a depth of colour that the 

 mixture, if made by rotating sectors, would appear white or 

 grey. This screen is used for viewing. To make the negative 

 another exactly similar screen is placed in front of the plate, 

 but the colours of the lines on this differ, just as Mr. Ives' 

 coloured screens for taking the negatives differ from his viewing 

 screens. 



When the negative is taken, a transparency is made from it, 

 and the viewing screen is placed behind it, so that the red line 

 covers the place through which the orange negative was taken, 

 and so on. Then, and not till then, the picture appears in its 

 natural colours. 



Instead of using transparencies and coloured films, transparent 

 inks may be used to produce pictures by three printings. 



The next process described was the oldest, namely, the pro- 

 duction of colour by the action of light itself. The present year 

 is its jubilee. Becquerel found that if, instead of iodising a 

 plate, he chlorinised it, and then exposed it to white light, it 

 gradually assumed a violet tint, and if, in this state, he exposed 

 it to the spectrum he was able to obtain the colours of the 

 spectrum on it. 



Abney, some years ago, showed that the red tint was due to 

 the lavender-coloured material taking up oxygen, whilst at the 

 violet end the subchloride became further reduced : thus the big 

 molecules formed by the addition of the oxygen vibrated slower, 

 whilst the abstraction of chlorine gave smaller ones vibrating 

 quicker. Since he was able to get the same effect on collodion 

 plates, it is not probable that the colours are due to stationary 

 waves, because, if so, they could only be viewed by reflected 

 light. Unfortunately, however, these colours, from the very 

 manner in which they were produced, were not permanent, and 

 no method has been devised for fixing them. 



The last method Captain Abney described of obtaining pho- 

 tographs which showed colour, but not coloured photographs, 

 was that of Lippmann, who found that if, by means of reflection, 

 he obtained stationary waves in the film, on development the 

 silver was deposited between the nodes. On reflecting light 

 from such a " noded " plate, the proper light alone was re- 

 flected, and the photograph, viewed at a particular angle, 

 appeared in its natural colours. If looked at by transmitted 

 light, these photographs have merely the appearance of ordinary 

 negatives. 



The proceedings closed with a vote of thanks to the lecturer, 

 proposed by Prof. Burdon Sanderson, and seconded by Mr. A. 

 Y. Walden. 



THE INTERNA TIONAL CONGRESS ON 

 TECHNICAL EDUCATION. 

 'PHE International Congress on Technical Education, opened 

 -*■ by the Duke of Devonshire at the Society of x\rts on Tues- 

 day, June 15, was continued on the three following days. Many 

 important papers were read, and there was a large attendance of 

 delegates from the continent and abroad. We have extracted 

 from the Times the subjoined brief reports of a few of the papers 

 read and the discussions which, took place upon them. 



