196 



KNOWLEDGE. 



M \v. I'Ml. 



of a glass cylinder, to the bottom of which is attached a piece 

 of glass capillary tubing, and then a piece of thin glass tubing. 

 bent so as to be vertical like the cylinder (Figure 1). 



A coloured solution 

 is put into the cvlinder, 

 and then by turning on 

 the tap it passes 

 through the capillary 

 and rises in the vertical 

 tube until the level is 

 the same as that of 

 the liquid in the 

 cylinder. The rate of 

 rise in the vertical tube 

 IS rapid at first, and 

 then becomes slower 

 .ind slower until the 

 level is attained. 



A measurement of 

 the time taken to rise 

 one centimetre, when 

 the difference between 

 the level in the tube and 

 that in the cylinder 

 was twehe centimetres, 

 gave fifteen seconds. 

 When the difference of 

 level was six centi- 

 metres the time was 

 thirty seconds, and 

 when the difference of 

 le\el was three centi- 

 metres the time taken 

 was sixty seconds ; so 

 that if H is the height 

 in the cylinder, and h 

 the height in the tube : 

 ]\ate of rise is pro- 

 portional to H — h, 

 or : Kate = Constant 



IH-h). 

 We see that for this 

 .apparatus H exactly 

 I finesponds with D oo, 

 the ultimate density of 

 a plate, and h corres- 

 ])iinds with D, the 

 density already at- 

 tained. 



Just as the constant 



for the glass apparatus 



depends on the size of 



the capillary, so the 



constant for development depends on the rate of penetration 



of the developer and the products of the reaction through the 



film, or the diffusion. 



In general, however, when we deal with the development of 

 a plate, we ha\e to deal, not with one exposure, but with many 

 different degrees of exposure received on different parts of the 

 plates and giving rise to different ultimate densities in these 

 parts. 



In order to see the effect of this we may construct another 

 simple apparatus (Figure 21 consisting essentially of three 

 separate apparatuses similar to that described above, three 

 identical tubes being arranged in front and connecting by means 

 of identical capillary tubes with three cylinders at the back but, 

 these cylinders are arranged so that the heights are variable, 

 and three different heights, HI, H2 and HJ, are given to the 

 three tubes. Now if we start this apparatus we shall find that 

 h2 rises faster than hi, and hi faster than h2. In fact, while 

 at first the levels in all three tubes were equal, after they have 

 risen the levels are very different, the level of h2 being above 

 that of hi, and h3 higher still. Figure 3 shows the apparatus 

 after the levels have risen. 



Transferring this to a plate with three different exposures 



IKt 



cap.able of giving ultimate densities Dl, D2 and D3. we see 

 that it will mean that while at first they are all e(|ual. as 

 development proceeds the contrast between dl,d2 and do will 

 iticrease until finally an ultimate contrast will be reached, 

 dependent on the value of 1)3. 



In order to measure the contrast between the levels in our 

 three tubes the most obvious thing to do is to draw a line 

 through the three points. The slope of this line will then be 

 a measure of the contrast, and clearly the rate of increase of 

 slope will be proportional to the difference between the 

 ultimate steepness attainable and the steepness already 

 attained. 



PHYSICS. 



P.y A. C. G. Kgkrtox, B.Sc. 



RAYS OF POSITIVE ELECTRICITY'.— Professor Sir 

 J. J. Thomson gave a lecture on a " New Method of Chemical 

 Analysis," on Friday, 7th April, at the Royal Institution. 



If an electric discharge is passed through a large vacuum 

 tube whose cathode is a tube of fine bore, positively charged 

 particles stream back from the cathode mo\ing in the opposite 

 direction to the cathode rays. Sir J.J. Thomson has made a 

 thorough investigation of these rays, which were called by 

 Goldstein, their discoverer, "' Kanalstrahlen." The apparatus 

 employed to study these rays, consists essentially of two glass 

 vessels connected by a narrow tube : the cathode is a fine bore 

 metal tube provided with an ebonite plug which fits the tube 

 connecting the two vessels. The one vessel is the discharge 

 tube, the other the observation tube ; the anode is situated 

 at the side of the discharge tube. The discharge tube is made 

 very large, so that a high potential discharge can be passed 

 through the tube at very low gas pressures without danger of 

 puncturing the glass tube. Cathode rays stream away from 

 the cathode into the discharge tube. " Kanalstrahlen " stream 

 through the fine tube, which constitutes a portion of the 

 cathode, back into the observation vessel. Here they ha\'e 

 to pass through a powerful magnetic field and electric field, so 

 arranged that the magnetic field deflects them vertically, and 

 the electric field horizontally. They impinge after undergoing 

 this deflection on to a photographic plate or a willemite screen ; 

 the former is affected by the rays and registers their position 

 and intensity, the latter by the phosphorescence set up gives a 

 visible impression of their deflection. 



The deflection of the rays is such that they may be divided 

 into two classes — primary and secondary rays. The primary 

 rays give short parabolic arcs having their heads in the same 

 vertical line, which shows that the minimum electrostatic 

 deflection undergone by the particles is the same whatever the 

 nature ot the gas. This means that the maximum potential 

 difference through which the particles have fallen is the same, 

 or that they take their origin close to the cathode. The 

 secondary rays are produced by the primary rays in their 

 passage through the gas in the observation tube, and their 

 position of formation can be found by altering the distance 

 through which the primary rays are under the influence of the 

 magnetic and electric fields. Sir J.J. Thomson has found in 

 this way that the secondary rays are due to dissociation of 

 systems in the undeflected Kanalstrahlen, due to collision with 

 negatively electrified corpuscles, giving rise to a negatively 

 electrified and a positively electrified portion. The reverse 

 can also take place within certain velocity limits, namely the 

 collision of a positive ray with a negative corpuscle and 

 consequent production of an tmcharged particle. The study 

 of the shape of the curves produced by the action of the 

 particles on the photographic plate shows that either of these 

 two actions may occur. 



Sir J. J. Thomson, in his lecture at the Royal Institution, 

 pointed out how the results of this investigation may be 

 applied to develop a new method of chemical analysis, which 

 is applicable to minute quantities of vaporisable substances, 

 and which, without the necessity of using pure materials, will 

 give directly the atomic weights of the substances under 

 investigation. For the relative positions of the ends of the 

 small parabolic arcs due to the primary rays are proportional 



