Mr. W. J. Lewis's Crystallographic Notes. 119 



subject of investigations recently by Mr. Herbert Tomlinson 

 and Professor Silvanus Thompson. The conclusion reached 

 by both is that the diminution of resistance is really due to 

 the contact between the electrodes ; and it appears that Pro- 

 fessor W. F. Barrett has arrived at a similar conclusion, as a 

 result of experiments made upon a " button of compressed 

 lampblack." Without knowing any thing about the nature of 

 these experiments, the writer desires to record his belief that 

 this theory does not entirely account for the facts stated above. 

 Besides, it seems a little difficult to understand how so small 

 a pressure as fifty grams, added to an already existing pres- 

 sure of about the same amount, can increase the area of con- 

 tact between a flat plate and a flat disk nearly four times, to 

 say nothing of the "recovery" which takes place so promptly 

 upon the removal of the pressure. 



XIII. Crystallographic Notes. By^SY. J. Lewis, M. A.* 

 [Plate HI.] 



pSEUDOBROOKITE.—A pupil of mine, whilst making 

 a list of the apatites in the Brooke collection preparatory 

 to their registration, called my attention to a specimen of 

 asparagus-stone from Jumilla, Murcia, on which were some 

 minute black crystals of apparently rhombic symmetry. They 

 were clearly not hematite, which is frequently found in thin 

 laminas in the matrix from this locality. The measurements 

 obtained show it to agree well with the mineral discovered by 

 Dr. Koch (Groth's Zeitschrift f. Krys. iii. p. 306), which he 

 has called pseudobrookite. 



The crystals are very small, and consist of simple prisms 

 with the makrodiagonal pinakoid, strongly striated parallel 

 to their intersections, and terminated by minute bright dome- 

 planes (figs. 1 and 2). They have a considerable tendency to 

 more or less parallel growth j and the opposite planes of the 

 prism are not, as a rule, accurately parallel. They are brittle, 

 and seem to have no good cleavage. 



The quantity- so far obtained from the specimens in the Cam- 

 bridge collection is very small (not more than one grain), and 

 it has been impossible to get a chemical analysis made. A 

 preliminary examination by Dr. Hugo Miiller confirms, to a 

 certain extent, the belief that it is identical with Koch's pseu- 

 dobrookite. The following table gives the angles observed 

 and calculated, as also the corresponding angles given by Dr. 

 Koch:— 



* Communicated bv the Crystallolosical Society, having been read 

 June 3, 1882. 



