1012] on Vital Efert.s of Radium and other Rays. 301 



l<'i-oin some experiments with the " active deposit " from thorimn, I 

 think that the action of these rays upon the skin must be very sHght. 



Beta Rays. — The beta rays consist of what arc called corpuscles, 

 being extremely minute material particles about one thousand times 

 smaller than the alpha ray, Lravelling at varying velocities from half 

 that of light, or less, almost up to the velocity of light itself, which 

 is 1S6,()00 miles per second. They carry a negative charge of 

 electricity, and their power of penetration varies with their velocity, 

 l)nt all of them are stopped by lead one centimetre in thickness. 



Gamma Rays. — The gamma rays, which are almost certainly a 

 wave disturbance in the ether, are similar in that and in other 

 respects to the ;z;-rays, l)ut they are of much greater penetral)ility, 

 passing easily through one centimetre of lead. No Crookes tube, 

 even with the highest vacuum, could give rays as penetrative as the 

 gamma rays produced spontaneously by radium. 



[Show sparkler burning.] 



Emitting as it does these three kinds of rays, radium will affect 

 a photographic plate, cause a diamond to shine in the dark, give a 

 sensation of light to the human eye, and make barium platino-cyanide 

 and willemite fluoresce. It has definite biological effects, and it 

 will also discharge an electroscope. I have already described the 

 electroscope, which enables us to measure the strength of a particular 

 specimen by comparing the rate of fall of the leaf with the similar 

 action in the case of some standard quantity. 



[Show radium tube, containing 20 mgr. of pure 

 radium bromide. Show also radium lent by Mr. 

 Francis Fox. 



Slide showing magnetic deviation of alpha and 

 beta rays. 



Slide : Different penetrability of a, ^, -y rays. 



Slides : (1) The " A " slide, showing the feeble 

 penetrability of the alpha rays, and the absence of 

 effect on a photographic plate when a piece of paper 

 was interposed. 



(2) Slide showing the darkening of a plate exposed 

 to gamma rays through a lead box one centimetre in 

 thickness. 



Effect of radium on platino-cyanide screen and 

 willemite. 



Discharge of electroscope under radium rays.] 



Reaction. — Becquerel's accidental discovery that the presence of 

 a tube of radium in the waistcoat pocket brought about a severe 

 burn on the skin, demonstrated the physiological effect of this 



