﻿752 Dr. T. J. Baker on Breath Figures. 



The analysis given above immediately applies ; we get 

 from (11), (13), (14), and (19) 



6' jl = t — s/er = sin 28/r . p 2 —q 2 /2pq. 



In the case of magneto-striction we can write ^ = 1 + 6! 

 and ^ = l-+-e 2 , where e l and e 2 are small. The difference 

 between the position of the principal axis before and after 

 1 he strain can be neglected so that B = u, and we get 



0'= sin 2a (ei — e 2 ) l/r. 



This is the formula given by Knott and verified experi- 

 mentally by the author *. 



The author desires to express his obligation to Prof. 

 W. McF. Orr, F.R.S., for his interest in and criticism of this 

 paper. 



LXV. Breath Figures. By T. J. Baker, D.Sc. (Lond.)]. 



IF one breathes upon a sheet of glass which has been 

 cleaned with soap and water and polished with clean 

 linen, water-vapour condenses uniformly on the glass in such 

 a manner that the surface as seen by reflected light appears 

 dull and rather white. If the tip of a small blowpipe-flame 

 is caused to traverse the surface of such a plate and the plate 

 is then breathed upon as soon as it is cold, a whitish con- 

 densation appears on those parts which the flame has not 

 touched, whilst the track of the flame is marked by a form 

 of condensation which, owing to its transparency, appears 

 black by contrast with the neighbouring parts. 



This and certain allied phenomena were described by 

 Aitken J in 1893, and several letters discussing the subject 

 appeared in the pages of ' Nature ' § during the period 1911 

 to 1913, but no general agreement as to the cause was 

 reached. 



A lens shows that the white portion of the deposit consists 

 of lens-shaped drops which are isolated from each other,, 

 whilst the black condensation consists of a continuous film of 

 water. 



* Knott, Trans. Roy. Soc. Edin. vol. xxxv. p. 388; Hackett, Proc. 

 Hoy. Dab. Soc. vol. xv. (n. s.) p. 416. 



-j- Communicated by the Author. 



t Aitken, Proc. Rov. Soc. Edin. p, 94, 1893. 



§ < Nature,' May 25," June 15, July 6, 1911, Dec. 19, 1912, Feb. 6; 1913. 

 See also vol. vii. of Lord Rayieigh's ' Collected Scientific Papers.' 



