246 ]Mr John llattray on the Evolution of 



in sea-water, so as to remove all loose muddy or sandy 

 materials ; it was then completely submerged, and allowed 

 to remain at rest for some time, in order to get rid of all 

 bubbles of air that might be entangled among the more 

 minute ramuli before being finally placed ii)side the jar. 

 This last process was effected in the usual manner. The 

 jar was filled very slowly with sea-water, so as to prevent 

 small bubbles of air from being included, and so accumu- 

 lating at the top of the collecting tube when the jar 

 was reversed. The specimen w^as now slipped under and 

 inside this, without being allowed to reach the surface of 

 the water, and a second cylindrical vessel with a flat bottom, 

 and having a diameter a few inches greater than that of 

 the jar, its height being equal to about one-third of the 

 latter, was slowly filled with sea-water, and passed under 

 the vessel v/hich now contained the plant or plants. The 

 whole apparatus was then raised out of the water, and 

 exposed to the direct action of the rays of the sun. 



The time of this exposure w^as not always the same, 

 because in many cases, in which small specimens had to be 

 dealt with, a longer period had to be allowed, in order that 

 a sufficient amount of gas might be collected to enable 

 further observations to be made ; but in all instances the 

 periods of exposure have been carefully recorded, and the 

 conditions of the weather during those periods given. 



The changes of the temperature of the water in the 

 apparatus now introduced a disturbing element of great 

 importance, inasmuch as with its variations the amount of 

 oxygen, and of the other accumulated gaseous substances, 

 absorbed by it would also vary. To counteract as far as 

 possible these changes, or, in other words, to maintain the 

 uniformity of the temperature of the water in the vessels, 

 a current was established. This was readily effected by 

 pumping water at frequent intervals from the sea into a 

 tank at a higher level, from which it was again drained off 

 into the apparatus by means of a small india-rubber tube 

 provided with a small glass nozzle or a wooden clip, to 

 prevent the passage of a current of too great strength. 

 By this means the temperature of the water in the 

 apparatus was found to vary within very small limits ; but 

 I have been unable to maintain an absolute constancy, 



