Retrospective Criticism. 491 



beach at Walton, on the coast of Essex, in great quantities, 

 and a considerable business was there carried on in manu- 

 facturing that mineral, by a chemical process, into sulphate 

 of iron, or, as it is commonly termed, green copperas ; and 

 I think that it is most probable that the pyrites collected at 

 Sheppey is sent to London to undergo the same process. The 

 copperas works at Walton have been for some years past 

 abandoned, and the ground on which they formerly stood is 

 occupied by the buildings of the new town ; but the making 

 of copperas from pyrites collected on the beach there is 

 within the recollection of many of the present inhabitants of 

 Walton. The uses to which copperas is put in the arts, &c, 

 are well known. 



Whence is the Calcareous Spar derived which is found more 

 than a Quarter of an Inch thick in the Argillaceous Nodules , or 

 Septaria ? (p. 381.) — I beg to submit that this spar is derived 

 from the water, charged with calcareous matter, which is in- 

 filtrated through the substance of these nodules into the cracks 

 formed in them by desiccation. The colour it appears of is 

 merely accidental. The spar in these nodules is sometimes 

 dark brown, at others of the colour of amber. These no- 

 dules are sometimes cut and polished, and are very beautiful, 

 owing to the various colours of the spar. — John Brown. 

 Stanway, near Colchester, Essex, Aug. 2. 1836. 



An Enquiry for the Cause of the Difference in Temperature 

 of River Water and Spring Water, both in Summer and Winter. 

 In the summer time the river water is much warmer than 

 that from a spring ; during the severe frosts of winter it is 

 colder; and when the stream is covered over with ice, the 

 spring, that is, well or pump, water is unaffected by frost. 

 Does this difference proceed from the exposure of the surface 

 of the river water, in summer, to the sun's direct influence ; 

 and, in winter, to that of frost ; while the well water, being 

 covered, is protected from their power ? Or is there in river 

 water, from the earthy particles it contains, a greater suscep- 

 tibility of heat and cold? — Indigena. April 19. 1836. 



[The answering of Indigena's enquiry is left to the favour of 

 other correspondents. Considerations more or less relative 

 to the subjects of enquiry are in V. 91. 303. 395, 396, 770. : 

 they are offered on the subject of anchor frosts.] 



Art. XIII. Retrospective Criticism. 



Remarks on the recent Controversies between C. Waterion, 

 Esq., and the Rev. F. O. Morris. — It must be a source of 



