34 BEPOET — 1889. 



found, some years ago, to be not above 800" C. The recent admirable 

 investigation of this point by Dr. J. Hopkinson has shown it to be 680° 

 C, rising to 712° C. during recalescence. Dr. Hopkinson has also shown 

 that the temperature at which steel ceases to be magnetic is 690° C, so 

 that he has definitely established the important fact of the identity of the 

 two temperatures. 



When we remember that the mechanical, electrical, and thermal pro- 

 perties of iron and steel are of the utmost practical importance, and that 

 all these properties appear to undergo a remarkable change at the critical 

 temperature, the need of a searching investigation ou the question is 

 obvious. Moreover, the interest is no less, from a theoretical point of 

 view, in connection with theories as to the molecular structure of magne- 

 tised iron and steel. 



Recalescence being feeble in soft iron, but marked in hard iron and 

 in steel, it -was conjectured that the phenomenon might be due to an 

 action depending on the presence of carbon in iron, creating an effect 

 analogous to that observed when water or super-saturated solutions are 

 cooled below the solidifying point ; a slight disturbance of the liquid 

 thereupon producing a sort of explosive action, solidification occurring 

 with a sudden rise of temperature. Professor Roberts- Austen has drawn 

 attention to the fact that lately M. Osmund, in Prance, has made recal- 

 escence the starting-point of a new inquiry, tracing the efi'octs of Carbon, 

 Mn, Cr, S, P, and Si, on the points at which recalescence occurs or is 

 destroyed by the alloy. That whilst pure iron does undergo a molecular 

 change at a red heat, carbon retains its position as an important factor in 

 determining the point of recalescence in hard iron and steel. Messrs. 

 Barus and Strouhal have recently published in America a series of impor- 

 tant papers on the tempering of steel, and shown that a critical tempera- 

 ture of between 500° and 1,000° C. exists, which is intimately connected 

 ■with the hardening or annealing of steel. They also draw attention to 

 the numerous phenomena observed to occur at this temperature. Numerous 

 other experimenters have worked at recalescence, notably Mr. Tomlinson, 

 of King's College, and Mr. Newall, at Cambridge, but the bibliography 

 and discussion of the whole subject will be reserved to the report next 

 year. 



Report of the Committee, consisting of Mr. John Murray (Secre- 

 tary), Professor Schuster, Sir William Thomson, the Abbe 

 Eenard, Mr. A. Buchan, the Hon. R. Abercromby, and Dr. 

 M. Grabham, appointed for the Collection and Identification of 

 Meteoric Dust. 



Several of the members of this Committee have had informal meetings, 

 and collections of dust have been received from several important stations 

 in Oceanic Islands. Others are expected soon, and it is believed a detailed 

 report can be drawn up before the next meeting of the Association. No 

 money grant is required. 



