140 Progress in Science. [January, 
once comes off. The pinch-tap, c, is now closed, and the compressed air in 
the Wolff's bottle still keeps the acid in the adapter. When it is desired to 
stop the flow of gas, the pinch-tap, c, is opened, the compressed air escapes, 
Fic. 16. 
and the acid in the adapter falls, leaving the gas-generating materials dry. 
Should the blowing of air by the mouth be deemed objectionable, an air-ball 
may be attached to the pipe, B. A safety-tube, c, is used, in order to prevent 
Fic. 17. 
the liquid from the wash-bottle being drawn over on stopping the supply 
of gas. 
ErkaTA.—Owing to postal delays the following errata did not arrive at the 
printing office in time for correction :—Page 2, line 3 from top, for “external” 
read “nocturnal.” Page 5, line 2 from top, for “Mars” read “mass.” 
Page 35, lines 2 and 18 from bottom, for “ Jan. 22nd, 1872” read “ Nov. 13th, 
1$71.”’ 
