550 PHILOSOPHICAL TRANSACTIONS. [ANNO 17QQ. 



cined magnesia; but, though they sprouted, the leaves never rose above the sur- 

 face, and the plants were entirely without roots : nor did they appear to grow better 

 in magnesia which had been washed in water containing fixed air. Calcined mag- 

 nesia was however much more destructive, as the seeds would not come up in it. 

 To compare its effects on vegetables with those of lime, each of these earths was 

 mixed, in different proportions, with sand, in small cups, in which seeds were 

 then sown. The lime was obtained from marble; and, before it was put into the 

 sand, was made to fall to powder, by being moistened with water. In a mixture 

 of 4 oz. of sand with 3 or 4 gr. of calcined magnesia, it was a long time before 

 the seeds came up, and the plants had hardly any roots or stalks; and with lOgr. 

 or more of magnesia, there was no appearance of vegetation. Thirty or 40 gr. of 

 lime did not retard the growth of the seeds more than 3 or 4 gr. of magnesia, and 

 the injurious effects were not so lasting. The lime, by absorbing fixed air, soon 

 lost its destructive properties; so that, after keeping these mixtures 4 or 5 weeks, 

 seeds were found to grow in that with 40 gr. of lime, nearly as well as pure sand; 

 but, in that with 5 gr. of magnesia, they produced only the seed-leaves, as before 

 described. It was necessary occasionally to break in pieces the sand which had so 

 much lime, as it would otherwise have been too hard to admit the seeds to pene- 

 trate through it. Plants will bear a much larger proportion of magnesia in vege- 

 table soil than in sand: with 20 gr. however of calcined magnesia, in as much soil 

 as was equal in bulk to 4 oz. of sand, the seeds produced only the seed-leaves, 

 without roots; and with about 40 gr. they were entirely prevented from coming up. 

 In countries where the magnesian lime is employed, it was said that the barren- 

 ness of any spot on which a heap of it had been laid, would continue for many 

 years. To learn how far it could by time be deprived of its injurious qualities, I 

 procured some pieces of mortar made of this species of lime, from 2 houses, 1 of 

 which had been built 3, and the other 8 years: they were taken from the outside 

 of the building, where they had been exposed to the air. After they were reduced 

 to powder, seeds were sown in them. Only a few came up, these produced merely 

 the seed-leaves, without any roots. As plants would grow in the limestone from 

 which this species of lime was formed, though not in the mortar made from it, I 

 wished to know what proportion of the fixed air originally contained in the lime- 

 stone, had been absorbed by the mortar. For this purpose, a piece of it was 

 finely powdered, to render it of a uniform quality: it was then tried how much of 

 this powder and of the limestone would saturate the same quantity of acid: by 

 this means, I ascertained the proportions of limestone and mortar containing equal 

 quantities of the magnesian lime. The fixed air being obtained from them in 

 those proportions, and measured in an inverted vessel, with quicksilver, it was 

 found that the mortar which had been exposed 3 years had absorbed 43, and that 

 of 8 years, only -i^ths of the quantity originally contained in the limestone. I 

 was not able to obtain any mortar which had been made earlier, though it might 

 deserve to be known how much fixed air it was ultimately capable of absorbing. 



