the Rocks of the Transandine Tunnel. 

 Preparation of Solutions. 



37 



Owing to the extreme sensitiveness of the electroscopes 

 used, and the small quantity of material dealt with at each 

 experiment, every possible precaution was taken throughout 

 to avoid the possibility of any errors due to contamination. 

 The work was carried out in a room which had never con- 

 tained radioactive preparations of any sort. The apparatus 

 was — with few safe exceptions — new, and hitherto unused. 



Extreme care was taken to avoid the introduction of radium 

 from the use of impure chemicals, it having been observed in 

 previous work that the commercial alkalies and hydrochloric 

 acid may contain a sufficient quantity of radium to exert a 

 noticeable effect upon the electroscopes. Both the water and 

 the hydrochloric acid used were distilled in the laboratory. 

 In the latter case, the distillation was performed over com- 

 mercially pure chloride of sodium, so that 100 c.c. of purified 

 acid, evaporated down, showed no trace of sulphuric acid with 

 barium chloride; certain experiments having shown that traces 

 of sulphuric acid are capable of diminishing the emanating 

 radioactive power of rook solutions. 



Care was taken in the preparation of the specimens for 

 chemical treatment that they were not exposed to any risks 

 attendant upon handling, but were in nearly all cases mani- 

 pulated by forceps. The quantities of chemicals used, 

 together with a full description of the electroscopes and of 

 the method employed, may be found in ' Eadioactivity and 

 Geology/ chap, xii., and need not therefore be further 

 described. 



Considerable difficulty was experienced in obtaining 

 solutions free from precipitate ; often three, and sometimes 

 even four, refusions having been made. It was, however, 

 frequently found that a repetition of the fusion failed to 

 render soluble the original precipitate. It may be noted that 

 those solutions which contained precipitate, and which in the 

 table given below are distinguished with a letter p, appear to 

 show no falling off from the general mean. 



Calibration of the Electroscopes. 



Particular attention was paid to the calibration of the 

 electroscopes. Of these, two were continually in use, " A" 

 being calibrated to an alkaline, and " B " to an acid solution. 

 Both were of about 620 c.c. capacity, and much alike in the 

 dimensions of the gold-leaf system. The earlier calibrations 

 were effected by an observation of the rate of collapse of 



