September, 1911. 



KNOWLEDGE. 



357 



rarely be found too long for a landscape subject with a close 

 foreground. 



When, however, there is no close foreground in the subject 

 and little deep shadow, the range of contrast becomes very 

 much less, it is quite possible for the intensity ratio between 

 the darl<est and lightest parts of an open landscape to be 

 only one to three, and in this case it is absolutely necessary 

 that care should be taken to keepontheside of under-ratherthan 

 over-exposure, because, whereas with contrasty subjects the 

 aim of the photographer must be to compress his scale in order 

 to get it within the scale of the printing paper, with flat subjects 

 having little range of contrast the object of the photographer 

 must be to increase the contrast in order to get an effect 

 pleasing to the eye. even though not necessarily a perfect 

 rendering of the original. One effect of this desirability of 

 increased exposure for increased contrast is found when such 

 unusual subjects as tropical or alpine landscapes are photo- 

 graphed. In the tropics, with vertical sun and the absence of 

 cloud, there is very little sky light to illuminate the shadows, 

 and the shadows are consequently extremely deep and show 

 much contrast with objects illuminated by the direct rays of 

 the sun. This increase of contrast compensates almost com- 

 pletely for the greater intensity of illumination, and it is 

 probable that a plate in the tropics should be given quite as 

 much exposure for the same type of subject as would be 

 required in England. Such photographs of the tropics as 

 have come before my notice seem to bear this out very fully ; 

 they are nearly all under-exposed, in spite of the great 

 intensity of illumination available. In the same %vay, the in- 

 tensity of the light in the high alps is largely counter-balanced 

 by the increased contrast due to the lack of sky illumination 

 in the shadows, so that skilled alpine photographers almost 

 always advise that plates in the high alps should be as fully 

 exposed as if the camera were being used in England. 



The relation between contrast and exposure does not seem 

 at present to have been fully worked out ; but there is no 

 doubt in my mind that it is a subject which will well repay 

 investigation, and that a discussion of practical photography, 

 based on a number of experiments checked by direct measure- 

 ment of contrast and intensity, would be of very considerable 

 educational value for all classes of photographers. 



PLATINOTVPE PRINTING. — Of all photographic 

 printing processes, the Platinotype process is recognised to be 

 that which gives results of the greatest beauty and the most 

 absolute permanencj'. It is also an extremely quick process, 

 requiring little time for manipulation, and, provided the 

 exposure is correct, producing results of unvarying excellence. 

 Unfortunately, however, it is not a printing-out process, and it 

 is necessary to judge the depth of the printing from the 

 appearance of the faint image produced by exposure to light, 

 a matter which, while it presents no difficulties to the 

 experienced worker who is continually doing it. is not so 

 easily undertaken by one who only prints in Platinotype at 

 rare intervals, and who is likely in the meantime to lose his 

 sense of the necessary printing depth. 



The use of mercury vapour lamps, however, both for light- 

 ing purposes and, as quartz lamps, for experimental work in 

 scientific laboratories, puts within the reach of every possessor 

 of such a lamp an almost perfect method of making platinotype 

 prints. Mercury vapour lamps are almost always regulated 

 either by hand resistances controlled by an ammeter or by 

 iron wire ballast resistances enclosed in hydrogen, and 

 consequently the intensity of illumination is very uniform and 

 the prints can be exposed entirely by time. At twelve inches 

 distance from a single Cooper- Hewitt tube taking three and a 

 half amperes, the exposure for an ordinary negative will be 

 about ten minutes, and a number of negatives can, of course, 

 be placed side by side under the long tube at one time. A 

 quartz lamp taking the same current at one hundred and ten 

 volts would require about five minutes' exposure at twelve 

 inches. This suggestion may be found useful by those 

 workers who have a, mercury vapour lamp a\'ailable. 



PHYSICS. 



By .\I.FRED C. G. Egerton, B.Sc. 



"SPHEROIDAL STATE." — When a little water is 

 allowed to drop on to a hot plate of metal heated to 300°C 

 about, it is well known that small spherical drops are formed 

 which run about on the surface of the metal ; these finally 

 either diminish in bulk owing to formation of steam or 

 collapse, produce a considerable quantity of steam and spread 

 out on the hot surface if the latter cools to a sufficiently low 

 temperature. The water is said to have gone into the 

 ■■ spheroidal state." The phenomena are well demonstrated in 

 a lecture by means of a silver dish perforated with a hole in 

 the bottom. The dish is heated by a burner, which is 

 removed ; a drop or two of water shaken on to the dish con- 

 tinues to move rapidly about on it until the metal has cooled 

 sufficiently when the drop resumes its ordinary condition and 

 can fall out through the small hole in the bottom of the dish. 

 The phenomena can be shown very distinctly by an 

 experiment of Sir James Dewar"s. A drop of liquid air is 

 allowed to fall on to small flat bottomed dishes containing 

 such liquids as sulphuric acid, ethyl alcohol, or carbon 

 tetrachloride. In each case the drop of liquid air assumes 

 the spheroidal state and moves rapidly about on the surface 

 of the liquid : but in the case of the more volatile liquids, such 

 as ethyl alcohol and carbon tetrachloride, the drop is 

 surrounded by a cloud of the condensed vapours of these 

 li(|uids and. as it mo\es about, is followed by a cloudy tail. 

 The drop impinging on to the walls of the vessel shoots off 

 again as if possessed of perfect elasticity. 



When the drop of water or liquid air first touches the hot 

 surface there is considerable production of vapour, which acts 

 as a cushion and prevents the liquid from coming into actual 

 contact with the hot surface. The liquid is then free to 

 assume the spherical drop form, which is due to the attraction 

 of the inner molecules upon the molecules residing upon the 

 outer skin of the liquid ; the drop then tends to become 

 spherical, so as to make the ratio of the surface to the volume 

 as small as possible. Imagine a little water to have been 

 allowed to fall on a hot plate ; there will be a sudden 

 production of steam, the water will disrupt into several distinct 

 drops which, since they are not actually touching the metallic 

 plate, will retain a spherical form, being buoyed up from the 

 plate by the cushion of steam. Since the under surface of the 

 liquid is heated most there will be more steam formed here, 

 and more molecules will leave the drop at the point nearest 

 the plate ; there will be then a continual disturbance of the 

 surface tension equilibrium along the surface of the drop, 

 tending to cause a rotation within the drop. The vapour pro- 

 duced will tend to shoot out from under one side more than 

 another, and so cause an initial movement in some particular 

 direction, and give a definite direction to the rotation of the 

 drop, which is maintained until some other disturbing influence 

 is met with. 



The motion of a particle of potassium or sodium when 

 thrown on to water is very similar to that of a liquid dropped 

 on to a sufficiently hot surface. The potassium causes a pro- 

 duction of hydrogen and also a considerable quantity of heat ; 

 some of the water is vaporised and no doubt the action is 

 carried on between the potassium and a thin jacket of steam, 

 the hydrogen produced acting as the supporting jacket for the 

 pea of potassium. 



The motion of the drop, its rotation, its collapse and the 

 curious palpitations it undergoes when rather too much is 

 added to form the true spheroidal state, are very prettily 

 illustrated by Sir James De war's experiment. The collapse of 

 the drop is no doubt due to the actual contact of the drop 

 with the hot surface, the surface tension equilibrium being then 

 completely upset, causing the drop to wet the surface : a 

 certain amount of heat is required to vaporise the liquid and 

 the hot surface may be cooled locally to such an extent that 

 insufficient steam is produced to support the drop and isolate 

 it from the plate. The palpitation of a drop spread on the 

 surface of a liquid is due to curious complicated changes in 



