344 Prof. McCoy and Dr. Yiol : Chemical Properties and 



hydrogen sulphide. After boiling out the excess of hydrogen 

 sulphide from the filtrate so obtained, this process was again 

 carried out, being repeated in all five or six times to free the 

 ThX from B, C, and D, all of which are precipitated with 

 the mercuric sulphide. After the last precipitation of 

 mercuric sulphide a few milligrams of barium chloride were 

 added to the solution and barium sulphate precipitated in the 

 boiling solution by means of sulphuric acid. The precipitate 

 settled in a few minutes, and was washed several times by 

 decantation and finally with alcohol. A very small amount,, 

 depending on the activity of the preparation but usually 

 about O'l mg., was then made into a thin film on a flat 

 metal plate. The time of the last precipitation of H 2 S was 

 taken as time zero, that is, the time when only ThX was 

 present. Barium sulphate carried down B and a part of the 

 C, the total amount of which formed during the time 

 between the precipitation of tbe last mercuric sulphide and 

 the barium sulphate was very small. 



In order to determine what amount of B and C remained 

 with the ThX after a series of precipitations of mercuric 

 sulphide, a "blank" experiment was made as follows. The 

 excited activity collected on a platinum dish was dissolved 

 in a few c.c. of hydrochloric acid, and this solution was 

 neutralized with pure ammonia, then made faintly acid ; the- 

 volume being about 20 c.c. In this six precipitations of 

 mercuric chloride were made, and then in the final filtrate a 

 little ferric chloride was precipitated as basic carbonate by 

 boiling the solution with an excess of sodium carbonate. 

 Basic iron carbonate carries down completely ThX as well 

 as B and C. The relative activities of the first four mercuric 

 sulphide precipitates were l'OO ; 0'026 ; 0*006 ; 0'002, the 

 first being about twenty times as active as the uranium 

 standard. The activity of the iron precipitate was 0'004,. 

 and this was found, from its rate of decay, to be due essen- 

 tially to ThX, which must have been carried to the platinum 

 dish by recoil. We therefore conclude that mercuric sul- 

 phide carries down B and C practically completely. 



A knowledge of the activity at time zero is also required ; 

 but this cannot be found directly, as about thirty minutes 

 elapse between the last precipitation of mercuric sulphide 

 and the first activity measurement. By means of the now 

 known value of the constant .r, it is found by equation (5) 

 that at t = 30 minutes, the initial activity should have dropped 

 to 0*998. Calculation also shows that at £=1*15 hours the 

 activity should exactly equal that at time zero. We have 

 therefore taken the observed activity at this interval after 



