GERMINATION — REQUISITES FOR IT. 347 



in chemical action or actinism, accelerate the process and cause rapid 

 growth. His experiments were performed by making the sun's rays 

 pass through different kinds of coloured glass. He believes that the 

 scorching effect of the sun on leaves may be prevented by the use of 

 blue glass, and that a high temperature might be obtained by red 

 glass. He has suggested a pale-green glass made with oxide of copper, 

 as that best fitted for conservatories. By this means he expects that 

 the scorching rays of light will be excluded, while no hindrance is 

 given to the passage of the others; the green colour being a compound 

 of yeUow or luminous, and of blue or chemical rays. A delicate 

 emerald-green glass has been employed, at his suggestion, in glazing 

 the large Palm-house at Kew. 



In order that plants may germinate vigorously, moisture, heat, and 

 air must be supplied in due proportion. If any of them are deficient, 

 or in excess, injury may be done. It is of great importance, therefore, 

 in agricultural operations, that the ground should be well pulverised, 

 the seeds regularly sown at a proper and equal depth, and the soil 

 drained. Pulverised soU, when examined, is found to consist of small 

 particles having cavities in their interior, and separated from each 

 other by interstitial spaces. In a very dry soil, all these cavities are 

 full of air ; in a very wet undrained soil, they are full of moisture ; 

 in a properly drained soil, the interstices are full of air, while the 

 particles themselves are moist. The seed in such a soil is under 

 the influence of heat, air, and moisture, and is excluded from light. 

 Hence it is in very favourable circumstances for germination. Great ■ 

 attention should be paid to the temperature of the soil in which seeds 

 are sown. Frost has an important eflfect in pulverising the soil, by 

 the expansion of the water contained in the particles, when it is con- 

 verted into ice. Snow, again, acts in giving a covering to the young 

 plant, protecting it from intense frost and sudden alternations of 

 temperature, and by its slow melting allows the plant to accom- 

 modate itself to the mild atmosphere. Snow contains often much 

 oxygen. 



If a field is not equally planted, the seeds will sink to different 

 depths, and will spring up very irregularly. In ordinary productive 

 soils seeds should be placed at a depth not greater than two inches. 

 Draining acts not merely in removing superfluous moisture, but in 

 allowing a constant renewal of nutritive matter, more especially of 

 ammonia and carbonic acid from the atmosphere, in giving a supply of 

 air, and in keeping up a proper temperature in the soil. In an 

 undrained soil the water is stagnant, and there is little supply of 

 fresh nutriment, and much cold is produced. There has been a dis- 

 cussion as to whether shallow or deep draining is the best. Much 

 depends on the nature of the soD, and it is impossible ,to lay down 

 any fixed rule applicable to all cases. Mr. Smith says that drains in 



