160 Intelligence and Miscellaneous Articles. 



optical axis, is covered on one side with tinfoil, and so placed under 

 the microscope that the surface of the tinfoil is visible, seen through 

 the crystal. If now the tinfoil be illuminated from above through 

 the crystal by sun- or lam]}-light, there are seen at a proper position 

 of the crystal a quantity of delicate brilliant rings, which are the 

 images of the reflecting parts of the tinfoil on the white light. The 

 delicacy of the rings renders the measurement tolerably accurate. 

 I found in this manner the width of the aperture of the cone, — 



For Tartaric acid, 4° 12' ; difference of the least and greatest 



observed values .... 34' 



4' 



3' 



2' 



li' 



For Arragonite I found 1° 52' (according to Lloyd, 1° 50' ; theory, 

 V 55'). Angstrom's coefficients of refi'action give for gypsum, 

 17' 25"; for nitre, theory givea 46' 25". — Poggendorff's Annalen, 

 vol. civ. p. 188. 



ON THE INFLUENCE OF PRESSURE ON CHEMICAL AFFINITY. 

 BY DR. LOTHAR MEYER. 



In the twelfth volume of Poggendorff's Annalen, there is a note by 

 Babinet which contains the proposal to use, as a measure of che- 

 mical affinity, the pressure which a gas generated by chemical de- 

 composition must attain in order that the decomposition may cease. 

 The author states that for zinc and sulphuric acid the limit is reached 

 ■when for 0° C. the pressure of the liberated hydrogen amounts to 

 thirteen atmospheres ; at 25° C, on the contrary, this pressure ex- 

 ceeds the height of thirty-three atmospheres. 



Experiments which I have made in Prof. Werther's laboratory do 

 not agree with these statements. With the most varied strengths 

 of sulphuric acid, even in the presence of large quantities of differ- 

 ent sulphates, and by the use of citric and acetic acids, the pressure 

 of the hydrogen liberated by zinc far exceeds the limits given by 

 Babinet. The reason of this appears to lie in the fact that Babinet 

 used copper vessels closed by a cock, while I used sealed glass tubes. 



The decomposition appears, however, to attain a limit; at any 

 rate the liquid, even with excess of zinc, has still a strong acid reac- 

 tion after standing for months. But what the maximum of this pres- 

 sure may be I have not been able to determine, inasmuch as the only 

 tubes I could obtain which would stand the pressure were too narrow 

 to allow a manometer to be introduced. The greatest pressure 

 which I observed directly at the manometer was 66 atmospheres. 

 The acid consisted of one volume SH- O^ and three volumes of H'^ O; 

 the temperature was 0° C. The tube exploded shortly after observ- 

 ing this pressure. — Poggendorff's Annalen, vol. civ, p. 189. 



