Mr. S. P. Thompson on Permanent Plateau's Films. 269 



The actual operation of correction will be the following : — 



1. The length of the correcting magnet must be parallel to 

 P. If it is to be applied end-on, it must be in the line P. 

 If it is to be applied broadside-on (which is preferable) draw 

 the line m m through and at right angles to P : the centre 

 of the magnet must be in that line. 



2. Join P C, and if necessary produce C to Q. Draw 

 R parallel to P C. Then Q R is the angle through which 

 the direction of the compass-needle is to be changed by the 

 application of the correcting-magnet ; and the distance of that 

 magnet is to be changed (always preserving its direction, as 

 already described) till the compass-needle points in the direc- 

 tion R 0. Instead of C, A or B might have been used in the 

 same way. 



Hoyal Observatory, Greenwich, 

 March 11^1878. 



XXXVIII. On Permanent Plateau's Films. 

 By Silvanus P. Thompson, B.Sc. B.A.* 



1. rj^HE film-figures, which occupy so large a part of the 

 -■- researches of Plateau t upon the Molecular Statics of 

 Liquids, when prepared with the glyceric fluid prescribed by 

 their discoverer, are of extreme fragility and of short duration. 

 With such a liquid films have been made which lasted ten, 

 twelve, or even sixteen hours in the air, and from fifteen to 

 thirty hours when protected by an external vessel of glass. In 

 one instance J, where chloride of calcium had been added to 

 the liquid, a duration exceeding fifty-four hours was observed. 

 The average duration of the films, especially if they are to be 

 exhibited to a number of persons, is more brief. 



No method hitherto described of producing these films in a 

 more durable or permanent form has been quite satisfactory, 

 though there have been several attempts. Of these the writer 

 was not aware when he began the present investigation, though 

 most of them are mentioned in the later chapters of Plateau's 

 work already named. A brief enumeration of these attempts 

 will therefore preface a description of the process now an- 

 nounced for rendering the films permanent. 



2. M. Plateau has himself endeavoured § to fix the film- 

 figures by dipping the wire frames into solutions which eva- 



* Communicated by the Physical So3iety. 



f Statique experimeMaU et theorique des liquides soumis mix seides forces 

 molecidaires. Par J. Plateau. Gand et Leipzic : 1873. 

 X Plateau, op. cit. vol. i. p. 175, § 106. 

 § Ibid. vol. ii. p. 119, § 311. 



