December, 1913. 



KNOWLEDGE. 



467 



having been used as an " antichlor " in neutralising the 

 bleaching agent used in the manufacture of the paper or the 

 material from which it was made, its presence may bring 

 about fading of the print. On this account mounts for silver 

 prints should be selected with considerable care. There are, 

 however, several delicate tests that may be applied which 

 will detect " hypo," even when present in very small 

 quantity, the following being one of them. Take about three 

 or four square inches of the card to be tested and tear it up 

 into small pieces and put these to soak in a glass beaker 

 which is about half filled with distilled water, after which add 

 about half a dram of pure strong sulphuric acid and a little 

 pure zinc. The beaker is then to be covered with a piece of 

 filter paper upon which a few drops of a solution of lead 

 acetate have been allowed to fall. The presence of " hypo " 

 will be indicated by the acetate of lead becoming brown from 

 the formation of lead sulphide, and if after the expiration of 

 about half an hour there is no appearance of stains, it may be 

 assumed that "hypo" is absent. In any case the rapidity 

 with which the stains are developed will depend upon the 

 quantity of " hypo " present. The test depends upon the 

 nascent hydrogen evolved by the action of the sulphuric acid 

 upon the zinc, reducing the sodium thiosulphate ("hypo"), 

 with the formation of sulphuretted hydrogen, which combines 

 with the lead acetate to form lead sulphide, the reactions 

 being expressed by the equations 



Na2S 2 3 + 4H, + H 2 S0 4 = 2SH a + Na 2 SO, + 3H a O 



Pb (CH 3 COO) 2 + SH.2 = PbS + 2 (CH 3 COOH) 



The presence of " hypo " in prints may readily be detected in 

 a number of ways, the iodide of starch test being commonly 

 employed for the purpose, the solution being made up in 

 the following manner. A small piece of starch is taken and 

 boiled in about three drams of distilled water until a clear 

 solution is obtained, to which when cold about a dram of iodine 

 dissolved in alcohol is added: this causes a dark-blue coloured 

 solution to be produced due to the formation of iodide of 

 starch. " The addition of iodine dissolved in water by the aid 

 of potassium iodide will answer instead of iodine in alcohol." 

 To apply this test to ascertain whether " hypo " is present in a 

 print, small pieces of the latter are boiled in a test tube 

 containing some distilled water ; the solution is then allowed 

 to get quite cold when one or two drops of the iodide of 

 starch are added to it, and should the blue colour be 

 discharged the presence of "hypo" is indicated, but should 

 the blue colour remain it may be assumed that this salt is 

 absent. The presence of " hypo " in the washing water of prints 

 is readily shown by taking two test tubes and filling one with 

 pure distilled water and the other with the water to be tested, 

 and if a drop or two of the iodide of starch solution be added 

 to each, sufficient to give a faint blue colour to one when 

 viewed by looking through it at white paper, a total absence 

 of any colour in the other will show that " hypo " is present 

 in it. The presence of " hypo " is also easily shown by means 

 of an alkaline solution of potassium permanganate, for which 

 purpose the following formula by Professor Bottcher is to be 

 recommended : — 



Potassium permanganate (pure) ... l£ grains 



Caustic soda ... 15£ ,, 



Water (distilled) 18i ounces 



When using this solution the presence of " hypo " is shown 

 by the pink colour changing to a green, owing to the 

 reduction of the manganese salts. The washing should 

 therefore be continued until a small quantity of the water 

 contained in a test tube will allow of some of the pink 

 solution being added to it without its becoming green. 



PHYSICS. 



By Alfred C. Egerton, B.Sc. 



SPARK PHOTOGRAPHS AT HIGH PRESSURES.— 

 The curious phenomena which attend the passage of 

 electricity through gases are manifold ; the nature of the effect 

 is chiefly dependent on the pressure. At ordinary pressures 



the usual spark discharge is obtained ; as the pressure 

 increases, so it becomes more and more difficult for the spark 

 to pass : the length of the air gap must be decreased, and 

 when the discharge passes a fatter spark is obtained and a 

 larger current is carried across the gap. At low pressures, 

 the discharge passes more readily until a certain lower 

 limit of pressure is reached, when the discharge passes again 

 with great difficulty. The discharge first appears as an 

 elongated spark; then as a straight but rather fuzzy strip; then 

 it fills the whole space between the two electrodes, and then 

 appears the Faraday dark space near the negative electrode, 

 which is surrounded by a luminous glow ; then, again, behind 

 this appears the Crookes dark space, where the negative 

 electrons are ejected normally to the surface of the electrode. 

 As this space begins to show, so the discharge usually breaks 

 up into a series of light and dark bands. Finally the Crookes 

 space fills the whole tube, and if exhaustion is carried very 

 much further the space refuses to carry any discharge of 

 electricity. 



The study of these phenomena has led to the wonderful 

 developments of physical science during the last twenty years. 

 The positive, the negative or kathode, and the X-rays and 

 the rays they in turn give rise to — secondary rays — were all 

 discovered during investigations to explain these electric 

 discharges ; indeed, much light has been thrown on the 

 nature of matter and its relation to electricity by such 

 work done mainly by Sir J. J. Thomson and those who 

 have been inspired by his genius. An atom of matter is 

 built up of negative charges of electricity, some free to 

 move as they will, others bound, some controlling the 

 chemical properties of the atom, others resident within 

 and taking no part in chemical actions. These negative 

 charges are held together by the rest of the atom which is 

 positively charged. Whether this charge is concentrated in a 

 central nucleus or whether it is distributed throughout the 

 mass of the atom is still an open question ; the positive 

 portion is associated with the chief portion of the mass of the 

 atom. The effects obtained in the electric discharge in gases 

 are mainly due to the action of charged particles (electrons) or 

 positively charged atoms on the molecules of gas in their 

 path. If the velocity of the charged particle is sufficient it 

 will " ionise " the molecule on collision with it ; that is to say, 

 it will split it into oppositely charged parts. The current is 

 carried at the lower pressures by these moving ions. 



The explanation of the discharge through gases at high and 

 ordinary pressures is dependent on similar considerations. A 

 discharge will pass through some gases more readily than 

 others ; through neon the discharge passes most readily. In 

 most cases of the conveyance of electricity through gases the 

 ions have to be produced by some kind of rays, but in the 

 spark discharge there are many more ions produced in a given 

 time in the neighbourhood of the electrodes than recombine in 

 the same time, and the discharge, as it were, "supports itself." 

 The differences in the ease with which the discharge will pass 

 through the gas depend on the amount of energy required to 

 ionise the molecules of the gas. Investigations on these matters 

 have been most ably carried out 'by Professor Townsend, of 

 Oxford. 



Amongst the mass of literature on these subjects we may 

 note a paper read before the Rbntgen Society in April, 1913, 

 by Professor A. W. Porter. The terminals of an induction 

 coil are connected to two electrodes, one on either side of a 

 photographic plate. The photographic plate is situated in a 

 dark chamber, in which the pressure may be raised up to ten 

 atmospheres. When the negative electrode is against the 

 sensitive side of the plate, at ordinary pressures, fanlike 

 impressions of the discharge are obtained on developing the 

 plate; but on raising the pressure the fanlike expansions 

 separate into two filaments. The negative discharge gives 

 figures with lines of discharge which are characteristic of the 

 positive spark lines for the same gas ; that is, a positive re- 

 bound must occur after the negative discharge has taken place. 

 The distinction between the negative and positive dis- 

 charges is very marked, but they are somewhat more 

 similar at higher pressures. The fanlike discharges are shown 

 to be due to the presence of nitrogen. The spark in oxygen 



