AN INTERESTING PSEUDOSOLID 293 



graphs taken with the cameras. This operation occupied per- 

 haps 20 seconds, after which a second increment of weight 

 could be added and the proceeding repeated. It was thus pos- 

 sible to make perhaps 6 measurements upon each solid within a 

 period of 2 minutes, during which the foam showed no deterio- 

 ration whatever.^ Experiments on Poisson's ratio were also 

 made by compressing cylinders of foam between a fixed plate 

 and a movable plate attached to a micrometer scale, the results 

 being recorded photographically. 



The photographs were made with the help of 2 powerful arc 

 lights equipped with reflectors and the most rapidly moving 

 shutters we could obtain ready made. The photographic expo- 

 sure therefore occupied perhaps a i/ioo of a second. 



The remainder of the process was mechanical. The plates 

 were developed and measured with great accuracy upon a com- 

 parator, the mean of 5 measured diameters constituting the 

 diameter which was used in each calculation. 



It will be seen by a glance at the accompanying table that the 

 results of these measurements afforded a greater accuracy than is 

 usually obtained upon the common solids of laboratory practice, 

 with which no more than i per cent, of displacement can be 

 attained. 



It was also possible to make a series of photographs of the 

 entire foam cylinder after successive increments of compression 

 and then by superposing the plates to obtain accurate traces of 

 the path of each component particle (bubble). A " composite " 

 photograph of this character is reproduced in PI. XIV. 



It was found that the lines of flow were parabolic {xy" = const.), 

 as they should be in a solid, according to theory, which is not 

 well illustrated by most experiments. 



It was also found that such masses of foam could be ruptured, 

 and that in this respect they behave sensibly like very rigid 

 solids, such as steel, cast-iron, or rock, in spite of the fact that 



1 The apparatus here described was obviously intended to furnish data for a 

 complete discussion of the elastic constants of the pseudosolid, including the 

 relation between force and displacement, but the lack of a stable pier in our labor- 

 atory made it impossible to carry out the latter measurements and was the imme- 

 diate cause of the suspension of the work until more favorable conditions should 

 be available. 



