40 Dr. Mac Culloch on a peculiar Appearance 



the instrument-maker, and remember that we have scarcely peeled 

 the paper from its surface ; these considerations should alone be 

 sufficient to check the presumption of the theorist, and set bounds 

 to the arrogance of hypothesis. 



AxtT. VI. On a peculiar Appearance' exhibited by Hoar Frost. 

 By J. Mac Culloch, M.D., F.R.S.E., &c. 



[Communicated by the Author.] 

 It has long been known that dew, as well as hoar frost, has a ten- 

 dency to attach itself to particular bodies, rather than to others, 

 and to rough surfaces in preference to smooth. Hence also it is 

 found that hoar frost, which, from its permanence, is more easily 

 examined than dew, is frequently deposited on acute edges and 

 points, when the flat surfaces of the same bodies continue bare. 

 I need not notice the speculations which have been entertained re- 

 specting the connexion of this phenomenon with electrical agency ; 

 but it is also obvious that the attachment of hoar frosts to strings, 

 edges, and points, bears a striking analogy to that which occurs 

 in many cases of the ordinary crystallization of salts from solution 

 in water, where the preference is so often given to bodies of these 

 forms as the first bases of attachment. 



The causes which influence this mode of disposition are as 

 much unknown as is every thing that relates to this mysterious 

 process. It is to little purpose to form conjectures, or to propose 

 hypotheses on this subject ; but it is not useless to record any 

 facts, which, by their accumulation and ultimate comparison, may 

 tend to throw light on it ; and, with this view, I transmit to you 

 the enclosed sketqh of a crystallization, or rather a deposit, of hoar 

 frost, which is remarkable for its singularity, and which has not, 

 as far as I know, been hitherto noticed by those who have paid 

 attention to these subjects. As it will tend to save much expla- 

 nation in words, and will at the same time render the appearance 

 in question much more intelligible, I send you the sketch precisely 

 as it was made on the iron railing of the door-way where I at first 

 observed it. 



