July, 1913. 



KNOWLEDGE. 



275 



more convenient time. The reason of its being necessary to 

 develop the print is owing to the light having acted upon the 

 iron salt only, and before the product formed is able to reduce 

 the platinum salt in contact with it it becomes necessary to 

 float the exposed paper upon a solution of either potassium 

 oxalate or a mixture of this with sodium phosphate, known as 

 developing salts. Using the former, a stock solution is made 

 as follows : — 



Oxalate of Potash 

 Water 



16 oz. 

 54 oz. 



For use one part of this is diluted with two parts of water. 

 If this bath is made slightly alkaline with carbonate of soda 

 or potash it gives slightly warmer tones, and if made slightly 

 acid by means of oxalic acid, a colder or bluish colour results. 

 In either case the alkalinity or acidity should only be such as 

 to just alter the colour of the test paper used. If developing 

 salts are used, then a stock solution of the following strength 

 is prepared : — 



Developing Salts ... ... $ lb. 



Water 50 ozs. 



And for use one part is taken to which is added one part of 

 water. It is claimed for this developer that it gives better 

 half-tones, as well as a colder tone generally. Whichever 

 developer is employed, the print is floated face downwards 

 upon it, the print being then turned over to watch the progress 

 of development, which should be complete in about thirty 

 seconds. The temperature of the solution should be from 

 60° to 100° F. If the solution be below 60° the deposit is liable 

 to become granular, while if too hot there is a tendency to a 

 brown and muddy colour, although under-exposed prints may 

 frequently be saved by use of a warmer solution for their 

 development. As soon as the desired result is obtained the 

 print is placed into a dish containing a dilute solution of 

 hydrochloric acid, the strength being — 



Hydrochloric Acid (pure) 

 Water 



1 oz. 



100 oz. 



The prints are allowed to remain in this bath for ten minutes, 

 after which they are transferred to a second bath of the same 

 nature, and finally to a third, remaining for ten minutes in 

 each. After this they are washed for about half an hour in 

 several changes of water, a little carbonate of soda being put 

 into the last washing water to ensure the removal of all trace 

 of the acid. If the prints on removal from the water are 

 placed between blotting boards and allowed to dry slowly, 

 they will be found to remain perfectly flat and ready for 

 mounting by any method that may be thought most desirable. 



PHYSICS. 



By Alfred C. G. Egerton, B.Sc. 



IS SODIUM RADIOACTIVE? — It is a somewhat 

 striking fact that potassium and rubidium are radioactive ; 

 that is, they emit spontaneously radiations consisting of 

 negatively charged particles, or 0-rays. The electrical effect 

 produced by these rays from potassium is only about a 

 thousandth of the effect produced by the radiations from an 

 equal weight of uranium. Experiments seem to show that 

 this small effect is due to /3-particles spontaneously emitted 

 from the potassium or rubidium atom, and that these atoms can 

 therefore be described as being radioactive. N. Campbell, 

 who made this interesting discovery, has continued his 

 researches on the feeble activity of substances and has 

 investigated what are termed the *-rays — rays which are 

 too feeble to ionise a gas and render it capable of con- 

 ducting electricity. The energy in ergs necessary to ionise 

 an atom is 4-2 X10" 11 ; hence if the velocity of the 

 electrically charged particle which collides with the atom is 



less than \/ 4 ' 2 X 10 ", i.e., 3-6 

 im 



second, where m (the mass of the electron) is equal to 



X 10" centimetres per 



6-5 X 10' i ' i (the mass of the electron bears the same ratio 

 to a grain of wheat as the latter does to the whole mass 

 of the earth), the electron will not be able to ionise the 

 atom, and can only be detected by the charge it carries and 

 can give up to a conducting body. The velocity of such 

 8-rays have been measured and are of the order 3 X 10" centi- 

 metres per second, or smaller than the velocity necessary to 

 ionise an atom of a gas through which they pass. 



Now sodium is in the group of elements which are similar 

 to potassium and rubidium, but no radiations are detectable 

 from it, neither does lithium give any measurable radiation. 

 It is curious that there should be these two elements, potassium 

 and rubidium — more or less light atoms as compared with the 

 heavy, unstable, radioactive elements such as uranium, radium, 

 and thorium — which show radioactivity and stand alone in 

 this respect. Such a fact raises the question whether most 

 elements do not emit characteristic radiations spontaneously 

 to a slight extent, but that the radiations are moving too 

 slowly, and the radioactive changes are proceeding too slowly, 

 both for the rays to be detected by their ionisation or for the 

 rays to be detected by the sum of their individual charges. 



Some evidence for the radioactivity of sodium has been 

 brought forward by F. C. Brown in Le Radium for October, 

 1912. Although no measurable radiation is emitted by it, yet 

 from certain geological considerations there seems to be some 

 reason for the view. Joly has calculated the age of the earth 

 from the salinity of the ocean and the amount of salt carried 

 down to it by the rivers. The value is seventy million years. 

 However, from the amount of helium or of uranium in rocks 

 a measure of the age of the earth is also obtained, and this 

 makes the figure four hundred and twenty million years. 

 Consequently, if the latter value is correct, there is too little 

 salt in the ocean or too much carried down at the present 

 time by rivers ; if sodium was a member of a radioactive 

 series of elements there would be too little sodium in the 

 ocean and too much in the rivers, and in support of the actual 

 fact the proportion of sodium to chlorine in the rivers would 

 be such that the sodium would be in excess, whereas in the 

 sea the reverse would be the case. 



ZOOLOGY. 



By Professor J. Arthur Thomson, M.A. 



BREEDING LEECHES.— M. Moshin Khan gives an 

 interesting account of the breeding of leeches in the United 

 Provinces of India. It is the occupation of a class of people 

 called "Chohra." In April or May selected leeches are put 

 in earthen pots with " hair-cleaning clay " and a little water, 

 and the pots are put out of reach of all disturbances; for 

 the leeches are very sensitive when laying their cocoons. 

 When these are formed and have hardened, they are picked 

 up and put into closed cups of clay, which are changed every 

 alternate day for a fortnight. Then the breeders help the 

 young leeches out by breaking the shells. Each cocoon con- 

 tains five or six young ones. The young leeches are reared 

 in fresh water, and the breeder gives them meals from his 

 own body. Those that are sold for medical purposes are said 

 to lose their power of breeding ; so special " seed leeches " 

 are put aside. A leech stops sucking at once if there is any 

 pus mixed with the blood of the patient. The Chohra cleans 

 a gorged leech by puncturing it dorsally near the head and 

 pressing the blood out from the tail forwards. 



AN INSECT PROTECTED BY ITS MEALS. — Dr. 

 A. Ch. Hollande, of Nancy, reports a very interesting case 

 of protective coloration. The flower-buds of one of the 

 mountain mulleins, Verbascum nigrum, are pierced in 

 autumn by the larva of a Curculionid beetle called Cionus 

 olens, which eats the violet hairs of the stamens. The violet 

 vegetable pigment (anthocyan) eaten by the grub passes 

 down the food-canal and, as usual, into the blood (in 

 some uncoloured form). It is carried to the fatty bodies 

 and accumulates there in numberless granulations, again of 



