630 SCIENCE PROGRESS 



kinetic energy of the ejected electron) re-emitted as a quantum 

 of K characteristic radiation, or perhaps as a quantum of each 

 of the K, L, M, etc., radiations. 



One of the results of recent research has been to confirm the 

 evidence afforded by earlier work that there are characteristic 

 radiations emitted by the light elements harder than the K 

 series. To these radiations Dr. Barkla gives the name J series, 

 and he has discovered their emission from carbon, nitrogen, 

 aluminium, and sulphur. Thus the J radiation emitted from 

 nitrogen is about as penetrating as the K radiation from the 

 much heavier element silver, while the J radiations from 

 aluminium and sulphur are more penetrating still. 



As regards the bearing of his work on the quantum theory, 

 Dr. Barkla states that though X-radiation may be, and is, 

 emitted by electrons singly or in groups as a continuous process 

 and in any quantity whatever, it is also emitted in quanta 

 from atoms when certain critical conditions are reached. 

 X-radiation is absorbed in quantities which are small compared 

 with a quantum in certain processes. In other processes it is 

 absorbed in quantities larger than a quantum of the primary 

 radiation. There is no evidence of absorption of X-radiation 

 in whole quanta. In fact, X-ray phenomena do not support the 

 quantum theory of radiation as generally understood. They 

 indicate that the quantum is a unit of atomic energy rather 

 than of radiation, and that this unit necessarily appears in 

 certain processes of radiation and absorption. For example, 

 the energy emitted at the expulsion of a K electron is a quantum 

 because it represents that unit of atomic energy which is re- 

 quired to bring a K electron into the position and state of an 

 L electron. 



PHYSICAL CHEMISTRY. By Prof. W. C. McC. Lewis, M.A., D.Sc. 

 University, Liverpool. 



Osmotic Pressure. — Although the concept of osmotic pressure 

 has played an outstanding part during the last thirty years 

 in the development of physical chemistry, it is a remarkable 

 fact that even at the present time no very general agreement 

 has been reached as to the actual mechanism of the phenomenon 

 itself. The view that osmotic pressure is a simple bombard- 

 ment pressure identical in nature with the pressure exerted 

 by a gas, the dissolved substance functioning as a gas and bom- 



