278 Correspondence. 



gilles) ; but it would be rather difficult to define the difference be- 

 tween a clay and an earth, for most earths are in a certain degree 

 plastic, as in the shape of mud. I am inclined to consider that a 

 clay, which when worked up with water feels greasy, when squeezed 

 in the hand. This kaolin is plastic to a certain degree, for I have had 

 it " thrown" by the common native potters into thin saucer-like 

 vessels, and I commonly make use of it in chemical analysis for cru- 

 cibles and muffles, but it does not feel greasy ; however, as this mineral 

 it now appears may become of some consequence in manufactories, 

 it will deserve consideration in a separate paper. 



With regard to your remarks on the causes of haze, at page 43, 

 I must admit that there can be no limit assigned to which parti- 

 cles of dust can be carried by the winds. While our information is so 

 small upon these subjects, it would be idle to sit down gravely to argue 

 upon a subject of this kind, and it is even difficult to think of a good 

 and plain fact upon which to shew my suggestion, that dust is not 

 the cause of haze in the air, is even probable. The best I can think of 

 at the moment are the following : — A few days ago, with the thermo- 

 meter about 80°, the wet thermometer depressed about 15°, and the 

 barometer below its mean, the day broke with a sky overcast with 

 hazy cirri ; that is, no clear blue sky was to be seen, and though there 

 were no visible clouds, yet the zenith appeared as if streaked with hazy 

 indistinct clouds, like what sailors call "mare's tails" when in a clear 

 blue sky. The wind was steady at N. W. and a light breeze in the 

 morning, which increased in strength as the sun approached the zenith. 

 The weather was unpleasant, and the air with a dry hazy appearance. 

 We have no definite philosophic terms which will express my mean- 

 ing of the appearance of the air, so that objects ten and twelve miles 

 round were indistinctly seen like the dome of St. Paul's, viewed from 

 Hampstead hill. A high range of hills, about forty miles off, was 

 invisible. In this state matters remained until noon, after which I 

 observed some few cumuli forming in the N. E. a little above the 

 horizon. These gradually increased in size and number, and in alti- 

 tude, forming an arch about 15° in altitude, and extending from 

 perhaps 20° in Azimuth on each side of N. E. In this state of 

 things, I observed that all under this arch the air was not so hazy as 

 in other directions, and that in the upper part of it some clear blue 

 sky was visible between some of the cumuli, and the range of hills, 

 which lies in a direction about East, was no longer quite invisible, but 

 the outline though still hazy was distinctly to be seen. I noted this at 

 the time as a strong fact opposed to the (what we will call for short- 



