326 Scientific Intelligence. 



taining foreign particles is suddenly expanded condensation 

 occurs about these particles which subsequently settle down in 

 drops of moisture upon a glass floor where they may be counted. 

 As water is known to condense on ions as well as upon dust par- 

 ticles the results obtained by this method cannot be very exact. 



(c) The soluble filter method. When soluble filters such as 

 collodion-wool or sugar are used the filter may be dissolved and 

 the number of trapped particles in a small measured volume of 

 the solution counted. The difficulty with this method is that the 

 suspended particles may themselves be soluble, or particles which 

 were aggregates would tend to subdivide, (c) The spray 

 method. The air may be drawn through a spray and the cap- 

 ture counted or weighed after evaporating the water. As it is 

 extremely difficult to wet dry dust this method cannot be con- 

 sidered very efficient. 



It is to be noted that while all these methods tell something as 

 to the matter deposited the relation as to the amount actually in 

 suspension is rather uncertain. An improved method devised by 

 Dr. J. S. Owens is free from many of the above objections. He 

 has found that when a very small jet of air properly humidified 

 is made to impinge at a high velocity upon a surface such as a 

 microscope cover glass, the foreign particles adhere to the glass 

 and a record is obtained which may be removed, examined, and 

 counted microscopically. By varying the conditions of the 

 experiment he was able to select the form of the orifice, the 

 velocity of the air and arrangement of apparatus which gave the 

 most satisfactory results. Also by subjecting a specimen of air 

 to dust extraction in successive cells a very definite knowledge of 

 the efficiency of the apparatus wias secured. For the details the 

 reader will naturally refer to the original paper which also con- 

 tains an extended record of observations made at Norfolk, Eng- 

 land, and a discussion of the probable sources of contamination. 

 This investigation was carried out for the Advisory Committee 

 on Atmospheric Pollution. — Proc. Boy. Soc, 101, 18, 1922. 



F. E. B. 



7. The Principles of Geometry ; by H. F. Baker, vol. I, pp. 

 VIII, 183. Cambridge, 1922 (Cambridge University Press).— 

 The purpose of the author is to place the reader in touch with the 

 main ideas dominant in contemporary geometry, or at least for 

 those parts which precede the theory of higher plane curves and 

 irrational surfaces. The present volume is entitled Foundations 

 and is devoted to the indispensable logical preliminaries. Its 

 method, however, is very different from the usual text book of 

 geometry, on the one hand, and the work of the logical rigorist, 

 on the other, in which the fundamental conceptions are analysed 

 into a sufficient and final number of axioms. Adopting the view 

 that a Science grows up from the desire to bring the results of 



