94 INTERFEROMETRY OF SPECTRA. 



screw 5. These are also measurable at the interferometer gage g, H being 

 the half-silvered and N an opaque mirror, as in figure 54. The subsidence 

 of pressure due to cooling adiabatic compression is, however, also measur- 

 able at g in terms of the delaying pressure. For we should have (v denoting 

 volume, k the bulk modulus) 



AT; Az> 8p 



- = A0; -- =f 



V V K 



8p 



or, apart from signs, A0 = , where 8p is the subsidence of pressure due to the 



ka 



cooling A0, after adiabatic compression. Hence the original equation takes 

 the form 



_ 



~ 



an equation for measuring the specific heat of constant pressure of the liquid 



at temperature 6 and pressure - -. The observations thus consist in 



2 



measuring Ap (in displacement) and 8p in interference rings, both at the 

 gage g. 



One may estimate the value of 8p per atmosphere of Ap, for alcohol, by 

 way of illustration. Here in c.g.s. units, 



&=i.2iXio 10 ; a=i.iXio" 3 ; 6 = 20; p = o.7p; ^=0.58 

 Hence 



i.2iXio 10 X(i.iXio' 8 ) 2 X2oXio 6 A dynes 



8p = - = i. 52X10* 



42Xio 6 Xo.79Xo.s8 cm. 2 



or 



5^ = 0.015 atm. per atm. of A 



Hence, if dp = 200 atmospheres, the above gage would show about 10 rings 

 for dp. Similarly a pressure dropping adiabatically from 1,000 atmospheres 

 would show about 50 rings, residually, after closing. 



The present research was planned to be pursued at much greater length; 

 but owing to the annoyances of a quivering pier, which are particularly 

 marked during the term weeks, and to the injury of the screw before the tal- 

 low washer was inserted, it was thought wise to discontinue it at present. 

 If the micrometer is to be set to icr 4 cm., the ellipses must be reasonably 

 quiet, and in case of long-distance interferometry such a condition can be 

 realized only during the summer months. 



