RADIOGRAPH 



the production of new atoms pos- 

 sessing different, yet distinct 

 chemical properties. The radiations 

 given off are parts of the disinte- 

 grating atom (hence the conserva- 

 tion of matter is preserved), and 

 they provide a measure of the rate 

 of breaking up of the atoms. The 

 new atoms formed may be unstable 

 and break up in their turn, giving 

 off a characteristic type of rays, 

 and this transformation process 

 continues through a number of 

 stages. No element of atomic 

 weight less than that of lead has 

 been found to be radio-active. 



Only about one atom of radium 

 in about 100,000,000,000 (one hun- 

 dred thousand million) breaks up 

 every second, or about half the 

 atoms in 1,800 years, and each dis- 

 integrating atom expels an a par- 

 ticle. The residual atom is called 

 radium emanation a heavy gas, 

 much more unstable than radium, 

 and transforming with the expul- 

 sion of an a particle to half -value in 

 about four days. A number of suc- 

 ceeding transition products are 

 derivedfrom the radium emanation, 

 denoted by the names radium A, 

 radium B, radium C, to radium F, 

 the transformation of each member 

 of the family being accompanied by 

 the emission of either a or /3 

 particles, or both. Each of these 

 transition products is to be re- 

 garded as a new element, although 

 some have only an average life of a 

 few minutes. 



The continuous production of 

 fresh radio-active matter by a radio- 

 element, and the continual decay 

 of the matter so formed, according 

 to a definite law, explains quite 

 satisfactorily all the known facts of 

 radio-activity. It is important to 

 emphasise that these changes go on 

 quite spontaneously, and cannot be 

 controlled by outside agencies in 

 any way they proceed at the same 

 rate, at temperatures varying from 

 that of liquid air ( 186 C.) to that 

 of an electric furnace (2,000 C.). 



All known radio-active substances 

 can be conveniently divided into 

 three families, the parent elements 

 of which are uranium, actinium, 

 and thorium ; radium itself belongs 

 to the uranium series. 



Radio-activity has brought us 

 very much nearer to the conception 

 of the nature of the atom. It has 

 introduced us to an electrically 

 charged atom of helium (the a 

 ray), which appeals to our physical 

 sense, inasmuch as it can be 

 detected individually, either by its 

 action on a fluorescent screen, as in 

 the " spinthariscope," .where each 

 atom (a particle) striking the 

 screen produces a visible pin-point 

 of light (" scintillations "), or by its 

 electrical effect by the movement 



6461 



of a delicate electrometer. See 

 Radio-active Substances and their 

 Radiations, Sir E. Rutherford, 

 1913 ; Radio-activity and Radio- 

 active Substances, Chadwick, 1921. 

 Radiograph. Name given to a 

 photograph made by means of X- 

 rays. It was found by Lenard in 

 1894 that the rays from a Crookes 

 tube affected a photographic plate 

 very much as light affected it, and 

 in 1896 Rontgen obtained a radio- 

 graph of the hand, showing that 

 flesh was almost transparent and 

 bones by contrast opaque to the 

 rays. The lodgement of foreign 

 substances in the body, the fracture 

 and diseases of bones, and even 

 digestive processes by suitable diet, 

 may be observed by means of radio - 



fraphs. The use of the method has 

 een extended to the study of the 

 internal structure of many sub- 

 stances, for taking finger prints, for 

 detecting adulteration in food, flaws 

 in machinery castings, etc. See 

 X-Rays. 



Radiolaria. Order of Protozoa. 

 They have a large number of 

 thread-like pseudopodia, and a 

 flinty external skeleton or shell, 

 often of great beauty. The single- 

 celled body is usually spherical or 

 conical in form, and the proto- 

 plasm is divided into inner and 

 outer portions by a kind of mem- 

 brane pierced with many pores, so 

 that both are in vital connexion. 

 All the Radiolaria are of micro- 

 scopic size, and their skeletons 

 form a considerable part of the 

 ooze which covers large tracts of 

 the ocean bed at moderate depths. 

 At greater depths the ooze is ex- 

 clusively radiolarian in character. 

 Reproduction takes place mainly 

 by fission ; but at other times two 

 kinds of flagellate spores are 

 formed, and it is probable that 

 these conjugate, thus exhibiting 

 both sexual and asexual propa- 

 gation. See Ocean. 



Radiometer (Lat. radius, ray ; 

 Gr. metron, measure). Name given 

 to an instrument invented by Sir 

 William 

 Crookes to 

 show motion 

 caused by the 

 action of light. 

 It consists of a 

 windmill with 

 four metal 

 vanes, each 

 brightly po- 

 lished or sil- 

 vered on one 

 side and black- 

 ened on the 

 other. The 

 four supporting 

 :inns are rur- 



Eadiometer invented rie * on a deli : 

 by Sir W. Crookes cately pivoted 



,J RADISH 



vertical rod, and the whole appar 

 atus is enclosed in a high vacuum. 

 When placed in the light the wind- 

 mill revolves with a speed depend- 

 ing upon the intensity of the light. 

 The action is probably due, how- 

 ever, to the difference of tempera- 

 ture of the blackened and silvered 

 sides, the former absorbing more 

 heat rays from the light and react- 

 ing on the molecules of the rarefied 

 gas left in the vacuum. 



Radish (Lat. radix, a root). 

 Genus of annual and biennial herbs 

 of the natural order Cruciferae and 

 genus Raphanus. The garden 



Radish. Bunches of two cultivated 

 varieties of the vegetable 



By courtesy of Sutton & Sons 



radish (Raphanus sativus), whose 

 native country is unknown, was 

 introduced to Britain in 1548. 

 Radishes prefer a light, rich soil, 

 and thrive best in one which has 

 been heavily manured for previous 

 crops. Hence, when grown on a 

 large scale, they should follow cab- 

 bages or other members of the 

 Brassica genus. The first sowing 

 in the open air should be made in 

 March, in a sheltered position, 

 and then, by making successional 

 sowings once every three weeks 

 until October, and subsequently 

 in a heated greenhouse, a crop of 

 radishes may be obtained all the 

 year round. * 



The seed must be sown very 

 thinly, as if planted too closely the 

 roots will be hot in flavour and 

 stringy. Radishes may be forced 

 in a hot-bed by covering the seeds 

 lightly and watering with tepid 

 water. The long-rooted radishes 

 are the best for early or spring 

 sowing, and the round or turnip 

 radishes for summer treatment. 



