MEMORANDA, 175 



on submitting; the slide to a fjentle heat, I found that the 

 moisture slowly retreated, leavinj; patches of the shell dry, 

 and with the markings as distinct as before. But what I 

 particularly desire to draw attention to is, that I have observed 

 the dry part of the shell is uniformly bounded by straiglit 

 lines, which are parallel to the two directions of least dis- 

 tance of the dots. This will, perhaps, be better understood 

 on referring to the figure, which is an enlarged diagram of a 

 portion of half the shell, and in which the portion in dots is a 

 careful copy of a dry part of the 

 shell, where the markings are 

 dearly seen ; whilst the shaded 

 remainder of the figure is intended 

 to represent the parts obscured 

 by the damp, in which only a 

 very slight trace of the markings 

 is visible. In the figure it is 

 evident that there are two lines, 

 AB and AC, in the directions of 

 which the dots are at a minimum 

 distance, and I find that the 

 straight lines of demarcation be 

 tween the moist and the dry portions are almost universally 

 parallel to these two directions.* 



Now upon the supposition of these little dots being eleva- 

 tions, the phenomenon appears to me easily explicable, on the 

 principles of capillary attraction. We can readily conceive 

 the moisture clinging from one dot to another, and it would 

 always have a tendency to arrange itself in lines parallel to 

 the directions of least distance. I am, however, quite at a 

 loss to imagine how the same principle would apply on the 

 hypothesis that the dots are depressions ; nor, on that hypo- 

 thesis, do I see upon what principle the phenomenon is expli- 

 cable. The examination has been made throughout with one 

 of Ross's recent l-8ths (1854), and a carefully centered achro- 

 matic ccmdenser, with stop to cut off tlie central rays. I always 

 observe that when I have the most distinct vision of the dots, 

 if I very slowly turn the fine adjustment so as to depress the 

 object-glass, the dots suddenly become white on a black 

 grountl, and under these circumstances I have sometimes 

 thought I could see the white dots having an hexagonal form ; 

 but even with the third eye-piece, I have not command of 



* From <2;eometrical considerations it is evident, that if the angle BAG 

 be greater than 120", the straight lines parallel to the straight line AD are 

 in the directions of the minimum distance of the dots, but by a careful 

 drawing with the camera lucida. I have found that the angle BAG is less 

 than 120". 



