354 PROCEEDINGS OF THE AMERICAN ACADEMY. 



higher degree than any other substance, the compressibility being as 

 low as any except the platinum metals, and the homogeneity in an- 

 nealed pieces of soft steel being nearly perfect. The use of a piezom- 

 eter of steel in determining the compressibility of mercury up to 6800 

 kgm. has been described in the paper cited above. The advantages of 

 the steel piezometer have been discussed there, and independent ex- 

 perimental evidence has been presented showing that the compressi- 

 bility of a piece of mild steel is the same in every direction, and that 

 therefore the piezometer does not change its shape. The results ob- 

 tained with this method were regular, and in so far justify the use of 

 this material ; but further work has led to the conviction that there 

 was present in the method a source of error for which it is hopeless to 

 attempt to calculate the correction. This error is concerned with the 

 leak past the freely moving piston ; this leak is less on increasing than 

 on decreasing pressure, so that the previous results are all too small. 

 It seems impossible to determine in what way the error so introduced 

 will vary with the pressure. The total magnitude of the error is prob- 

 ably between 2 and 3 per cent. 



The method adopted here attempts to keep the advantage of the 

 steel piezometer, while doing away with the uncertainty of the freely 

 moving piston. It was not possible to secure so regular results with 

 the new method as with the old, but on the other hand, any constant 

 source of error seems to be excluded. The method is a modification 

 of one used by Aimd ^^ in 1842, and since used in many modified forms 

 by other experimenters. In the original form, the liquid to be investi- 

 gated was placed in a glass bulb provided with a fine capillary com- 

 municating with a vessel containing mercury. The whole affair was 

 then lowered into the sea. Here the increase of pressure forced mer- 

 cury into the bulb, where it fell to the bottom and remained. On 

 withdrawing from the sea, the maximum volume compression was 

 measured simply by weighing the mercury forced in. Aimd's results 

 were very irregular and incorrect. The most obvious trouble with this 

 method is the tendency of the mercury to collect in drops at the mouth 

 of the capillary. On release of pressure, this drop flows back through 

 the capillary, and the resultant compressibility appears too low. The 

 difficulty may be minimized by making the capillary very small. This 

 is an easy matter when the capillary is made of glass, but when steel 

 is used as here, some special construction must be devised. The form 

 of piezometer adopted is shown in Figure 1. The upper piece B, 

 screws into the shell A. A tight joint between the two is made simply 



10 



Aira6, Ann. de Chim. et Phys., 8, 257-280 (1843), 



