April, 1912. 



KNOWLEDGE. 



159 



thus see that the absence of these rays, owing to the action of the 

 substance itself upon them, might be the cause of serious trouble 

 in photographing when the silver salt in use was mainly sensitive 

 to them, or the light particularly rich in such. It is thus 

 evident that the photographic rendering of certain bodies is 

 influenced by the nature of the plate employed and the kind 

 of light used in illiuninating the object. We will suppose that 

 a wash drawing has to be copied, and that it is illuminated by 

 the light from an enclosed arc, and that the negative is made 

 upon a wet collodion plate. Now the light itself is rich in 

 ultra-violet, and the wet plate very sensitive in this region. 

 The result is that a negative could be taken with a compara- 

 tively short exposure. If for the wash drawing we substitute 

 our white paper with the invisible writing upon it "made with 

 quinine sulphate " ([uite a strong image would be obtained 

 upon the wet plate owing to the substance having absorbed 

 those rays which produce a strong action upon the plate, with 

 the result that the sensitiveness of the silver salt has been 

 apparently destroyed in those parts, and a negative image of 

 the writing which will show as dark upon a more or less white 

 ground in the print, results. Sulphate of ([uinine, however, is 

 not the only body which behaves towaids light in this way. 

 Chinese white is a notable example and in the case of drawings 

 in which this is used and the photographs taken under the 

 conditions enumerated above, the effect is either to produce a 

 black or a very degraded white. Owing to this difficulty other 

 whites have been introduced, but unfortunately bodies are 

 present as well, which may complicate matters by setting up 

 chemical action, that may result in the white assuming a tint 

 which is photographically very non-actinic. That this is the 

 case can be \ery readily understood when we come to 

 consider the pigments with which drawings are made and 

 their close proximity and admixture with the white itself. 

 Then, again, the medium with which the white is ground may 

 exert some influence, and from some experiments made, this 

 appears to be the case, as pure oxide of zinc photographs 

 differently according to the method used in preparing the 

 pigment. If we use other sources of light for the illumination, 

 such as daylight or the open arc, again the results differ, so 

 that the only really satisfactory test as to the photographic 

 value of one of these pigments, is to make negatives under 

 precisely those conditions that obtain in practice. But even 

 then we shall only gain information as to how the white would 

 photograph when used under ideal conditions ; it will not tell 

 us what the result might be were some other substance 

 present in so small a quantity as not to visually alter its 

 colour and yet photographically it might do so to a con- 

 siderable extent. 



EXPOSURE TABLE FOR APRIL.— The calculations 

 are made with the actinograph for plates of speed 200 H and 

 D, the subject a near one, and lens aperture F16. 



PHYSICS. 



By .Alfred C. G. Egerto.n, B.Sc. 



VAPOUR PRESSURE.— When a substance is situated in 

 an enclosed space there is a certain pressure set up within that 

 space, owing to the tendency of the substance to vaporise. At 

 any particular temperature ail equilibrium is set up between 

 the molecules leaving the surface as vapour and those con- 

 densing back on the surface as liquid or solid. .As the 



temperature rises so the vapour pressure increases. Ice at 

 0" C has a vapour pressure of 4 ■ 6 millimetres of mercury ; 

 ice at 10° C, about two millimetres; water at 100°, seven hundred 

 and sixty millimetres. When the vapour pressure is the same 

 as the external pressure, then the liquid boils ; thus water 

 boils at 100° when the barometer stands at the normal pressure 

 of seven hundred and sixty millimetres. If the vapour pressure 

 of a solid is greater than that of the external pressure, then 

 the solid wmII vaporise without licjuefying. An example of this 

 is solid carbon dioxide, which has a vapour pressure, at so low a 

 temperature as — 60°,greater than the atmospheric temperature, 

 and goes over into gaseous carbon dioxide without licjuefac- 

 tion. Now gold boils at a temperature about 1000" C, that 

 is to say, its vapour pressm-e is seven hundred and sixty 

 millimetres at its boiling-point. As the gold cools, so its 

 vapour pressure decreases and becomes so small that, at 

 ordinary pressures, the vapour pressure is undetectable. It 

 would be very interesting if the vapour pressure curves of 

 metals could be carried down to very small values ; the 

 shapes of the curves would no doubt show whether the 

 molecules associated as the temperatures decrease and the 

 molecules of the metal become larger and heavier. It is known 

 that in solution in mercury the molecules of most metals are 

 simple atoms. It is difficult to conceive of iron, brass and 

 connuon metals of that sort having a definite vapour pressure 

 at ordinary temperatures and losing weight gradually — the 

 effect is so small as to be undetectable by known means : and 

 it is possible for a crystalline material of homogeneous com- 

 position that the molecules are so closely packed that 

 the vapour pressure is absolutely nil, and no molecules 

 whatever arc able to leave the surface. When the substance is 

 definitely crystallised and when the molecules possess so 

 little kinetic energy that they cannot leave the surface, one 

 would expect the vapour pressure to become zero. The 

 vapour pressure of mercury has been measured at tempera- 

 tures below 0° C and has a very small value (-0008), but the 

 pressure of the solid mercury is so small as to be unmeasur- 

 able. Vapour pressures of less than a hundred-thousandth of 

 a millimetre cannot be measured as yet. 



The methods of measuring small vapour pressures may be 

 sub-divided into two main classes — dynamic and statical. 

 The former class contains such methods as depend on the 

 passage of some inert gas over the material under investigation 

 and estimation of the amount carried over in a certain 

 time. The second class measures directly the actual 

 pressure set up by the vaporising substance. Some have used 

 a very thin copper vane dividing a vessel into two chambers. 

 Into one chamber the substance is introduced and the two 

 chambers are then evacuated and closed : any difference of 

 pressure is then measured by the deflection of the copper 

 vane, such deflection being detected by causing light falling 

 on the copper vane to interfere with the waves of another 

 beam of light. Another method, employed by Dewar and 

 others, depends on the change in rate of action of a " radio- 

 meter" as the pressure is altered. A radiometer is an instru- 

 ment which consists of a light vane mounted within a fairly 

 high vacuum — heat waves falling on one side of the vane, heat 

 that side and increase the motion of the gas molecules in its 

 neighbourhood and the vane is repelled away from them. Sir 

 William Crookes discovered this action and the experiments 

 led him later to the study of electric discharges in high vacua 

 and the discovery of the cathode rays. Lord Rayleigh devised 

 a sensitive manometer to measure small differences of pressure 

 —by tilting the manometer, the levels of liquid iri the two 

 limbs can be brought so as just to touch two very fine points 

 — the amount of tilt being measured by reflection of a spot 

 of light from a mirror. A method of measuring low pres- 

 sures of great ingenuity has been devised by Pirano. 

 This consists of a specially constructed electric filament 

 lamp of fine tungsten or platinum wire. The resistance of the 

 wire in one such lamp is balanced against that of another. 

 The one lamp is connected to the apparatus containing the 

 substance the vapour pressure of which is required; this 

 raises the pressure of the gas ever so slightly in the 

 one bulb, and hence heat is conveyed away from the wire 

 by the gas faster than from the wire in the other bulb 



