July 13, 1 882 J 



NATURE 



259 



reaction C0 2 + C = CO + CO shows that out of two volumes of 

 CO- we receive four volumes of CO, and it is accompanied with 

 absorption of heat, which is determined by the fact that the 

 combustion of one atomic weight of carbon develops 97 K (i.e. 

 97 great calorics, or 97,000 common ones), while the combus- 

 tion of CO develops 6S 4 K ; the reaction is thus accompanied 

 by the following thermal result : 97 o K - 2 x 6S'4 K= - 39 '8 K. 

 The result (97 - o-6S-4-68-4 = - 39-8 K) is the same for the 

 following reaction : H s O + C = CO + H 2 ; and, if the combustion 

 of hydrogen in the calorimeter were not accompanied by a 

 formation of liquid water, it might be admitted that the combus- 

 tion of CO and of H 2 develops the same amount of heat, which, 

 however, is not the case. 



After having shown how the conclusions on the heat of for- 

 mation of hydrocarbons from hydrogen and coal, or diamond, 

 are vitiated by not taking into account the heat developed, or 

 absorbed, by physical and mechanical processes, and how M. 

 Thomsen (Berliner Bcrichte, 1880, p. 1321) was brought to 

 erroneous conclusions as to the structure of the molecule of coal 

 and diamond, as well as to the structure of hydrocarbons ; M. 

 Mendeloef says : — " In using calorimetrical data of chemical 

 reactions to judge of the variation of chemical energy in a re- 

 action, it is necessary to free them from the influence of physical 

 and mechanical processes which accompany the reaction. Of 

 course, the relative influence of these secondary processes is not 

 very great, as the chemical process is the most important one, 

 especially in such energetic reactions as the combustion of hydro- 

 carbons ; but it is important, for strictly maintaining the prin- 

 ciple itself of thermo-chemistry, always to apply this correction, 

 as we always apply the correction for loss of weight in the air, 

 especially when weighing gases." "Only in the gaseous state 

 can we consider the thermal relations of bodies free from the 

 influence of the modified internal work, as was well pointed out 

 by Berthelot in the first chapters of his work : ' E-sai de 

 tie chimique' ; therefore, all comparisons must be made 

 in the gaseous state, as well for the bodies entering into reaction, 

 •e which we receive. When the determination of the 

 heat of combustion is made for solid or liquid bodies, we ob- 

 viously must add the latent heat of evaporation (and liquefac- 

 tion) of the body, and deduct the latent heat of evaporation of 

 water. This last is well known, and for a molecular weight in 

 grammes (18 grammes) of water, it is equal to about 107 K at 

 the temperature of 15 to 20° Cels. As to the heat of evapora- 

 tion of hydrocarbons, it is still not sufficiently known. But we 

 know that the heat necessary for the evaporation of moleettlar 

 quantities of different bodies comparatively volatile, varies from 

 4 K (as for NH 3 and N 2 0) to 15 K (as for quicksilver and ethyl), 

 and usually is between 6 K to 10 K." This correction not being 

 very great, and the determinations of heat of combustion not 

 being yet very accurate, Prof. Mendeleef takes, for those bodies 

 whose heat of evaporation is not yet determined, an approxi- 

 mate correction. Another correction is that which results from 

 changes of volume of combining bodies. The mechanical work 

 which results from this increase or decrease of volume is not 

 very great (0/57 K in most of the determinations of Thomsen), 

 but always must be taken into account. 



By applying these corrections, Prof. Mendeleef gives a new 

 corrected table of heats of combustion of twenty different 

 hydrocarbons, as well as the heats of formation of these bodies 

 from CMj, CO, and CO;. The corrections are not insignifi- 

 cant, as, for instance, for hydrogen, CH 4 , C 5 H e , C 3 H 5 , and 

 C r H lc , whose heats of combustion, as determined by Thomsen, 

 Berthelot, and Loughinin, are respectively — 6S4, 2I3'5, 373'5, 

 533"5, and II37'4; the corrected figures, as given by M. 

 Mendeleef, are— 57-4, 192, 342, 492, and 1062. 



THE WEDGE PHOTOMETER* 

 TV/T UCH attention has recently been directed to the use of a 

 A wedge of shade glass as a means of measuring the light of 

 the stars. While it has been maintained by various writers that 

 this device is not a new one, the credit for its introduction as a 

 practical method of stellar photometry seems clearly to belong 

 to Prof. Pritchard, director of the University Observatory, Ox- 

 ford. Various theoretical objections have been offered to this 

 photometer, and numerous sources of error suggested. Prof. 

 Pritchard has made the best possible reply to these criticisms by 

 measuring a number of stars, and showing that his results agreed 



1 Ey Prof. Edward C. PickeriDg. Presented May to, 

 Academy of Arts and Sciences. 



very closely with those obtained elsewhere by wholly different 

 methods. His instrument consists of a wedge of shade glass 

 of a neutral tint inserted in the field of the telescope, and 

 movable so that a star may be viewed through the thicker or 

 thinner portions at will. The exact position is indicated by 

 means of a scale. The light of different stars is measured by 

 bringing them in turn to the centre of the field, and moving the 

 wedge from the thin towards the thick end until the star disap- 

 pears. The exact point of disappearance is then read by the 

 scale. The stars must always be kept in the same part of the 

 field, or the readings will not be comparable. By a long wedge 

 the error from this source will be reduced. A second wedge in 

 the reversed position will render the absorption uniform through- 

 out the field. Instead of keeping the star in the same place by 

 means of clockwork, the edges of the wedge may be placed 

 parallel to the path of the star, when the effect of its motion 

 will be insensible. To obtain the best results, the work should 

 be made purely differential, that is, frequent measures should 

 be made of stars in the vicinity assumed as standards. Other- 

 wise large errors may be committed, due to the varying sensi- 

 tiveness of the eye, to the effect of moonlight, twilight, &c, and 

 to various other causes. 



A still further simplification of this photometer may be effected 

 by substituting the diurnal motion of the earth for the scale as a 

 measure of the position of the star as regards the wedge. It is 

 only necessary to insert in the field a bar parallel- to the edge of 

 the wedge, and place it at right angles to the diurnal motion, so 

 that a star in its transit across the field will pass behind the bar, 

 and then undergo a continually increasing absorption as it passes 

 towards the thicker portion of the wedge. It will thus grow 

 fainter and fainter, until it finally disappears. It is now only 

 nece-sary to measure the interval of time from the passage behind 

 the bar until the star ceases to be visible, to determine the light. 

 Moreover, all stars, whether bright or faint, will pass through 

 the same phases, appearing in turn of the 10, II, 12, &c, mag- 

 nitude, until they finally become invisible. For stars of the 

 same declination, the variation in the times will be proportioned 

 to the variations in the thickness of the glass. But since the 

 logarithm of the light transmitted varies as the thickness of 

 the glass, and the stellar magnitude varies as the logarithm 

 of the light, it follows that the time will vary as the mag- 

 nitude. For stars of different declinations, the times of 

 traversing a given distance will be proportional to the secant of 

 the declination. If 5, 5' are the declinations of two stars having 

 magnitudes m and /;;', and t, t' are the times between their tran- 

 sits over the bar and their disappearances, it follows that ;«' - m 

 = A(t sec S-t' sec S ). For stars in the same declination calling 

 A sec 5 = A' we have m-m = A'(l-l'). Accordingly the dis- 

 tance of the bar from the edge of the wedge is unimportant, 

 and, as in Prof. Piitchard's form of the instrument, it is only 

 necessary to determine the value of a single constant, A. Various 

 methods may be employed to determine this quantity. Prof. 

 Pritchard has recommended reducing the aperture of the tele- 

 scope. This method is open to the objection that the images 

 are enlarged by diffraction when the aperture is diminished ; 

 constant errors may thus be introduced. Changing the aperture 

 of a large telescope requires some time, and in the interval the 

 sensibility of the eye may alter. These difficulties are avoided 

 by the following method, which may be employed at any time. 

 Cover the wedge with a diaphragm in which are two rectangular 

 apertures, and place a uniformly illuminated surface behind 

 it. Bring the two rectangles into contact by a double image 

 prism, and measure their relative light by a Nicol. From 

 the interval between the rectangles and the focal length of the 

 telescope, the light in magnitudes corresponding to one second, 

 or A may be deduced. Perhaps the best method with a small 

 telescope is to measure a large number of stars whose light has 

 already been determined photometrically, and deduce A from 

 them. 



The great advantage claimed for this form of wedge photo- 

 meter is the simplicity of its construction, of the method of 

 observing, and of the computations required to reduce the results. 

 It may be ea-ily transported and inserted in the field of any tele- 

 scope like a ring micrometer. The time, if the observer is 

 alone, may be taken by a chronograph or stop-watch. Great 

 accuracy i> not needed, since if ten seconds correspond to one 

 magnitude, it will only be necessary to observe the time to single 

 seconds. The best method is to employ an assistant to record 

 and take the time from a chronometer or clock. If the stars are 

 observed in zones, the transits over the bar serve to identify or 



