422 



Hadio-activity of Uranium. 



nitrate by the ton, I have at my disposal some specially prepared 

 thorium nitrate, which is chemically pure. Thoria prepared from this, 

 tested on a sensitive plate, gave a feeble impression in 120 hours. 



38. In the present state of our knowledge of these radio-active 

 bodies it is safest to retain an open, or even a slightly sceptical, mind. 

 A¥e recognise them mainly by the photographic and the electrical 

 tests — reactions which are so sensitive that they give strong results, 

 even when the active body is present in too small a quantity to 

 be detected by its spectrum — one of the most delicate of tests. 

 Knowing the tendency of ordinary chemical bodies to be carried down 

 when a precipitate is formed in their presence, even when no question 

 of sparse solubility is involved, it is not surprising that radium and 

 actinium, to say nothing of UrX, appear to simulate elements which 

 may ultimately prove to be very different from them in chemical 

 €haracters. For instance, UrX dissolves easily in dilute sidphuric 

 acid, and, I have reason to believe, forms a soluble sulphate ; still, 

 when chloride of barium is mixed with it and precipitated as a 

 sulphate, I invariably find strong radio-activity in the precipitated 

 sulphate as well as in the filtrate from the barium sulphate. 



.To adduce a simile from my previous researches, the first surmises 

 as to the chemical characteristics of the bodies now known to be yttrium 

 iind samarium, were widely different from reahty. The differences 

 were entirely due to the perturbing cause which is active in the 

 present case — the tendency of the bodies to be carried do-\vn and 

 entangled in precipitates, where, according to ordinary chemical laws, 

 they ought not to occur ; and to the extreme delicacy of the radiant 

 matter test, which in the case of samarium detects one part in 2 J million 

 parts of calcium, and in the case of yttrium detects one part in the 

 presence of a million parts of extraneous matter. 



39. The radiographic test for these active bodies presents another 

 point to be borne in mind. Other tests for the presence of an element 

 either act quickly, or do not act at all, with a comparatively narrow 

 margin of debateable land where the indications of the test may be 

 doubtful. Here, however, the test is cumulative. Like an astronomer 

 photographing stars too faint for his telescope to disclose, he has only 

 to expose the plate for a sufficiently long time and the star reveals 

 itself on development. So, in the case of radio-active minerals or 

 precipitates, if no action is apparent at the end of an hour, one may 

 be shown after twenty-foiu- hours. If a day's exposure will show 

 nothing, try a week's. Considering my most active UrX does not 

 contain sufficient of the real material to show in the spectrograph, 

 yet is powerful enough to give a good impression on a photographic 

 plate in five minutes, what must be its dilution in compounds which 

 require an hour, a day, or a week to give an action 1 



