Appendix.] M. Thouin on the Use of the Scoria, 8$c. 21 



rejecting all pieces larger than a nut. The whole is beaten, to give 

 it solidity, though not sufficiently to make it quite compact. 



The plants being re-potted, after clearing their roots as much as 

 possible from insects, they are placed out ; and though the earth, 

 over which the Scoria is spread, be full of worms, not one penetrates 

 to the pots ; not only because they find no nourishment in the 

 super-stratum, but because the sharp, angular points of the Scoria 

 present insurmountable obstacles to their passage through it. 



The plants soon begin to shew the benefit derived from this 

 change, acquiring vigour, and growing with the greatest luxu- 

 riance. The plants treated thus by M. Thouin, are principally 

 those of the Cape of Good Hope and New Holland, belonging to 

 the Genera Protsea, Thymelea, Erica, and others. The same plan, 

 however, is adopted, with equal success, with such tropical plants 

 as are exposed to the external air in the height of the season ; 

 these were placed in a southern aspect, a little inclining to the 

 east, on a similar basis of Scoria. 



But as all these plants must necessarily be returned to the 

 house on the approach of cold weather, and some of them even be 

 placed on the tan bed, the insects with which the tan always 

 abounds, soon find their way into the pots, and their old ravages 

 upon the plants are renewed. 



After having made several trials, M. Thouin satisfied himself 

 that bottom heat was not necessary to plants after their second 

 or third year, but that the atmosphere of the house, if sufficiently 

 warm for the plant, was equally so for its roots : he therefore re- 

 jected the tan bed altogether, and supplied its place in the pit 

 with earth, upon which he laid a bed of Scoria, in the same way 

 as those in the open air, and upon the surface of this he placed 

 his pots. The change in the plants thus treated was soon per- 

 ceived, and was not less striking than it had been on those in the 

 open air. Consequently the expense of tan, and the labour 



