636 ADOLF F. VOIGT 



bonate precipitates. In either case the following relations between 

 weights and specific activities would be true: If Xi and X2 are the 

 specific activities of the added and separated samples and TFi and W2 

 are the weights of amino acid added and originally present in the mix- 

 ture, the relation: 



X2 ~ TFi 

 will hold. Solving this for TF2, the unknown weight , we obtain : 



(Xr - X2) 



W2 = Wx 



x. 



If the weight of added material Wi can be neglected in relation to Wo 

 or for a tracer available in some carrier-free form that will mix with 

 the substance in the mixture, the product WiXi can be replaced by the 

 observed total activity, Ai, of tracer added: the expression then 

 simplifies to TF2 = A1/X2. 



4. Radioaiitographs 



The radiations given off by radioactive isotopes permit a unique 

 scientific use of these isotopes, in that their position in tissue can be 

 located merely by allowing them to take their own picture. Such a 

 picture is called a radioautograph or autoradiograph. The method 

 was first applied to the gross location of the element in the plant or 

 animal being studied. Some examples of these are the studies by 

 Stout and others (54) of the uptake of phosphorus and of certain trace 

 elements in tomato plants (Fig. 12), which show, among other things 

 that concentration occurs in the seeds of the tomato. Other examples 

 of the technique are the studies using this method that have given bet- 

 ter understanding of the plant circulatory system. 



The real possibility of the method lies in its application to micro- 

 scopic studies. A radioautograph prepared in the usual way may 

 on microscopic study yield information about the location of the 

 tagged atoms within the individual cells. While most tracers can be 

 used for the preparation of macroradioautographs, application in 

 microscopic study places some restrictions on the usable isotopes. 

 The darkening of the photographic plate is, of course, restricted to 

 the paths of the various particles. Since the particles differ widely 

 in their penetrative power the length of their paths in the photo- 



