184 IODINE, SULPHUR, AND SULPHUROUS ACID. 



Wlicn apjilied lo living vegetables in the state of an exceedingly di 

 lute solution in water, it has been supposed upon some soils, and in 

 some circumstances, to be favourable to vegetation. Long experience, 

 however, on the banks of the Tyne, and elsewhere, in the neighbour- 

 hood of the so-called alkali* works, has proved that in the state of va- 

 pour its repeated application, even when diluted with much air, is in 

 many cases fatal to vegetable life. 



Poured in a liquid stale upon fallcnv land, or* land preparing for a 

 crop, it may assist the growth of the future grain, by previously forming, 

 with the ingredients of the soil, some of those compounds which have 

 been occasionally applied as manures, and which we shall consider 

 hereafter. 



Chlorine is represented by CI, and muriatic acid byHCl. 



II. — IODINE. 



Iodine is a solid substance of a lead grey colour, which, when healed, 

 is converted into a beautiful violet vapour. It exists in combination 

 chiefly with sodium, as Iodide of Sodium, in sea water and in marine 

 plants ; but it has not hitherto been detected in any of the crops usually 

 raised for food. 



Like chlorine, it is poisonous both to animals and plants; and was 

 found by Davy to assist and hasten germination. It may possibly exert 

 some hitherto unobserved influence upon vegetation, when it is applied 

 to the soil in districts where sea-ware is largely collected and employed 

 as a manure. 



Iodine is slightly soluble in whaler, and this solution has been men- 

 tioned in a previous lecture (VI., p. 107), as affording a ready means 

 of detecting starch by the beautiful blue colour it gives with this sub- 

 stance. 



III. — SULPHUR, SULPHUROUS AND SULPHURIC ACIDS, AND SUL- 

 PHURETTED HYDROGEN. 



1°. Suljjlmr is a substance too well known to require any detailed 

 description. In an uncombined state it occurs cbiefly in volcanic coun- 

 tries, but it may sometimes be observed in (he form of a thin pellicle on 

 the surface of stagnant waters — or of mineral springs, which are natu- 

 rally charged with sulphurous vapours. In this slate it is not known 

 materially to influence the natural vegetation in any part of the globe. 

 It has, however, been employed with some advantage in Germany as a 

 top-dressing for clover and other crops to which gypsum in that country 

 is generally applied. The mode in which it may be supposed lo act 

 will be considered hereafter.* 



2°. Sidphurous acid. — When sulphur is burned in the air it gives off 

 a gaseous substance in the formof white fumes of a well known intensely 

 suffocating odour. These fumes consist of a combination of the sulphur 



* In these works carbonate of soda (the common soda of the shops) and sulphate of soda 

 (glauber salt) are manufactured from common salt, and in one of the processes immense 

 quantities of muriatic acid are given ofTfrom the furnace, and used to escape into the air by 

 the chimney. 



1 The refuse heaps of the alkali works on the Tyne contain muoli sulphur and more gyp- 

 sum—but the farmers, perhaps, naturally enough, consider that if the works themselves do 

 harm to their crops, the refuse of the works cannot do them much good. There are thou- 

 sands of tons of this mixture which may be had for the leading away. 



