1908] Recent Researches in Radio-activity. 27 



WEEKLY EVENING MEETING, 

 Friday, Januaiy ol, 1908. 



^The Right Hon. Lord Kayleigij, O.M. P.C. M.A. D.O.L. 

 LL.D. Sc.D. Pres.R.S., in the Chair. 



Professor Ernest Rutherford, M.A. LL.D. D.Sc. F.R.S. 



Recent Researches in Radio-activity. 



In 1904 I had the honour of giving an address at the Royal Institu- 

 tion on the subject of Radio-activity. In the interval steady and 

 rapid progress has been made in unravelling the tangled skein of 

 radio-active phenomena. In the present lecture I shall endeavom* to 

 review very shortly some of the more important advances made in 

 the last few years, but as I cannot hope to mention, even briefly, the 

 whole additions to our knowledge in the various branches of the 

 subject, I shall confine my attention to a few of the more salient facts 

 in the development of which I have taken some small share. 



In my previous lecture I leased the explanation of radio-active 

 phenomena on the disintegration theory put forward in 190o by 

 Rutherford and Soddy, which supposes that the atoms of the radio- 

 active bodies are unstable systems which break up with explosive 

 violence. This theory has stood the test of time, and has been 

 invaluable in guiding the experimenter through the maze of radio- 

 active complications. In its simplest form, the theory supposes that 

 every second a certain fraction (usually very small) of the atoms 

 present become unstable and explode with great violence, expelling 

 in many cases a small portion of the disrupted atom at a high speed. 

 The residue of the atom forms a new atomic system of less atomic 

 weight, and possessing physical and chemical properties which 

 markedly distinguish it from the parent atom. The atoms com- 

 posing the new substance formed by the disintegration of the parent 

 matter are also unstable, and break up in turn. The process of 

 degradation of the atom, once started, proceeds through a number of 

 distinct stages. These new products formed by the successive dis- 

 integrations of the parent matter are in most cases present in such 

 extremely minute quantity that they cannot be investigated by 

 ordinary chemical methods. The radiations from these substances, 

 however, afford a very delicate method of qualitative and quantitative 

 analysis, so that we can obtain some idea of the physical and chemical 

 properties of substances existing in an amount which is far below the 

 limit of detection of the balance or spectroscope. 



