294 SCIENCE IN SHORT CHAPTERS. 



it would be a great advantage. The action of a little pent on the top 

 of a wall to protect fruit trees is very simple, and the explanation 

 was afforded by the experiments of l)r. Wells on dew. The frosts, 

 which did the greatest mischief, were due to radiation from the 

 ground on clear nights ; and it would be found that if one thermom- 

 eter were placed in a garden under an umbrella, and another on the 

 open ground near it, the differences of temperature would be very 

 considerable ; on cloudy nights there was very little difference. 

 Last night there was only a difference of 2, but a few nights before 

 it was 6. The period of greatest cold might not probably be more 

 than an hour, but it would be sufficient to do a great deal of mis- 

 chief, and anything which would check the radiation would have the 

 required effect. In the case of loosening the fruit trees from the 

 wall there was, probably, a double action ; it prevented the tree 

 being forced on by the warmth of the wall in the daytime, and also 

 avoided the chilling effect at night, a rough wall being a good radia- 

 tor, and sinking to a low temperature. He did not think there was 

 much danger to be apprehended from wind, because the canvas being 

 so open, the wind would pass freely through it ; but he had not seen 

 it subjected to any violent gale. 



CHAPTER XLIII. 



SOLIDS, LIQUIDS, AND GASES. 



THE growth of accurate knowledge is continually narrowing, and 

 often obliterating, the broad lines of distinction that have been drawn 

 between different classes of things. I well remember when our 

 best naturalists regarded their " species " of plants and animals as 

 fundamental and inviolable institutions, separated by well-defined 

 boundaries that could not be crossed. Darwin has upset all this, and 

 now we cannot even draw a clear, sharp line between the animal and 

 vegetable kingdoms. The chemist is even crossing the boundary 

 between these and the mineral kingdom, by refuting the once posi- 

 tive dictum that organic substances (i.e. the compounds ordinarily 

 formed in the course of vegetable or animal growth) cannot be pro- 

 duced directly from dead matter by any chemical device. Many of 

 such organic compounds are now made in the laboratory from mineral 

 materials. 



We all know broadly what are the differences between solids, 

 liquids, and gases, and, until lately, they have been very positively 

 described as the three distinct states or modes of existence of matter. 

 Mr. Crookes suggests a fourth. I will not discuss this at present, but 

 merely consider the three old-established claimants to distinctive 

 existence. 



A solid is usually defined as a body made up of particles which hold 

 together rigidly or immovably, in contradistinction to a fluid, of which 

 the particles move freely over each other. " Fluids " is the general 

 term including both gases and liquids, both being alike as regards the 

 mobility of their particles. At present, let us confine our attention to 

 liquids and solids. 



