Dec. 14, iS 



SCIENTIFIC NEWS. 



61 



could make the flint very much more transparent than 

 it would otherwise be. But for most purposes it was 

 desirable to make a thin slice or section of the flint, and this 

 was done by a very simple contrivance. If we take a piece 

 of glass, and having broken off a convenient piece of flint, 

 one side was ground down flat and smooth on some 

 powder, like emery or other grinding material. Having 

 done that, the smooth side of the piece of flint was stuck 

 on a piece of glass and then turned over, and the other 

 side of the flint ground until nearly all of it was ground 

 away, and there was nothing left but a thin film of 

 the rock, of which they wished to take the sec- 

 tion. In practice, by using a lapidary's wheel 

 they might shorten the task. He would now see 

 if it was possible to show some facts about the 

 microscopic structure of flint which would aid them in 

 reasoning concerning the nature of the origin of flint. 

 He would first endeavour to show some sections of 

 chalk, a substance in which, as they learned in the last 

 lecture, flint was always found. [Sections of chalk ex- 

 hibited.] Sometimes, when the sections did not give 

 them the information they desired, they might see what 

 was the nature of the chalk by washing it with a stiff 

 brush, when they could separate the number of minute 

 shells of which it was made up, and study them in- 

 dividually. On the screen were shown a number of 

 straight, wedge-shaped fragments, and there were a 

 number of rounded bodies which had the forms of 

 shells. He would endeavour to show them what 

 was the nature of the globigerina-ooze which re- 

 sembled chalk. Most of them could see it had those 

 peculiar forms of shells that he exhibited to them the 

 other day. There were a number of globular bodies 

 united together, making up these shells. The darkness 

 of those objects was due to the fact that the specimen ex- 

 hibited was fresh ooze from the bottom of the Atlantic, 

 and the dried- up bodies of the animals were in the shells. 

 [Lantern slide : showing shells of great variety of forms.] 

 When the sections of the chalk were carefully examined 

 by the aid of the microscope, all the objects were found 

 which were found in the mud at the bottom of the At- 

 lantic. [Picture of objects taken from surface of water 

 during Challenger Expedition.] Many of them were 

 soft-bodied creatures which left no trace behind them, 

 but the shells dropped to the bottom of the ocean, and 

 formed the mud of which he had spoken. Now he must 

 call their attention to the fact that there was one con- 

 siderable difference between globigerina ooze and chalk. 

 Hitherto he had dwelt on the close resemblance there 

 was between globigerina ooze and chalk. Globigerina ooze 

 differed from the chalk, however, in the fact that when they 

 dissolved the chalk in an acid, such as vinegar, they got 

 very little residue, and that residue seldom presented 

 any definite forms. Here and there was found a trace of 

 some wonderful structure composed of the substance 

 silica, which was not dissolved by ordinary acids, but 

 generally there were no traces of such organisms ; but 

 if they dissolved most kinds of globigerina ooze in acid, 

 there was left behind a considerable number of skeleton 

 organisms composed of the unsoluble substance silica, 

 and these siliceous organisms composed of silica be- 

 longed to three sorts of organisms : first the plant known 

 as diatoms or diatomacese; secondly, animals called 

 radiolarians ; and thirdly, the spicules found in the 

 sponges. Those sponges, like Venus's flower-basket, had 

 skeletons made up of silica. In the diagram they had 

 last week they had before them some wonderful varieties 



of structures which existed among those three forms of 

 life — diatoms, which were plants ; radiolarians and 

 siliceous sponges, which were animals. [Specimens shown 

 by lantern of siliceous organisms which exist in the ooze 

 at the bottom of the Atlantic Ocean, but which were not 

 found in chalk.] This was a very beautiful object, and 

 that which looked like vegetable was an accumulated 

 series of organisms which he had spoken of as diatomacas 

 and they required a very powerful microscope to under 

 stand them. [Further lantern slides of a white powdery 

 material, formerly largely dug for making dynamite, 

 because it absorbed the nitroglycerine better than any 

 other substance which had been tried were (hen exhi- 

 bited. [Specimens were shown of material dredged 

 by Challenger from the bottom of the Antarctic 

 and South Pacific Oceans. Also specimens of radio- 

 larians ; and the lecturer spoke of the wonderful 

 diversity of form and size they represented.] This was 

 material dredged from the bottom of the Atlantic, but in 

 many cases rocks were found made up entirely of these 

 organisms. A work on the subject of these radio- 

 larians had been published by Prof. Haeckel, of Jena, 

 in which he described 4,000 forms, and said that he had 

 far from exhausted all those brought home by the 

 Challenger Expedition. [Numerous specimens of radiola- 

 rians and transparent rods, which were spiculesof sponges, 

 wonderful forms presented by siliceous organisms which 

 went to build up the spicules of sponges ; some formed 

 meshes consisting of six branches. These objects were all 

 composed of silica.] The next thing which he had to call 

 their attention to was that the silica comprising these 

 organisms was all in the condition of colloidal silica, and 

 there was a test, as he had pointed out, by means of 

 which theycould distinguish colloidal silica from crystalline 

 silica. Colloidal silica, as they had seen, was remarkable 

 for the ease with which it passed into a state of solution, 

 and he should be able to show them that in many cases 

 these beautiful siliceous organisms, which were com- 

 posed of colloidal silica, were easily dissolved. First he 

 should endeavour to show by polarised light that those 

 objects were composed of colloidal silica ; and secondly 

 he should endeavour to show that in many cases those 

 objects had been dissolved, and were sometimes re- 

 deposited ; and that this colloidal silica passed over into 

 the crystalline condition. In the first place he would 

 endeavour to show them the fact that these objects were 

 really composed of colloidal silica, and not of crystalline 

 silica. That was shown by the fact that colloidal silica 

 did not affect polarised light. Therefore, if they attached 

 the polariscope to the microscope, and put the prisms 

 in such a position that it stopped off all light, these 

 masses of colloidal silica remained dark, whereas if they 

 consisted of crystalline silica they would get a coloured 

 light. [Specimens of sponge spicule; dissolved globigerina 

 ooze, the residue of which, however, did not contain 

 any of the beautiful forms of radiolarians or diatoms 

 or sponge spicules.] He wanted them to remem- 

 ber the fact that silica was, in the rocks they had 

 studied, in a state of solution, and that it was frequently 

 deposited from a state of solution, sometimes in a 

 colloidal condition, and sometimes in a crystalline condi- 

 tion. The other limestones of the globe, if dissolved in 

 acid, left behind a minute residue, and that residue, if 

 examined in the way they examined it now, was seen to 

 be composed of a great number of beautiful little crystals 

 cf quartz. Theysawnothingof the radiolarians etc. because 

 the silica had passed into a state of solution, and been 



