barrett] THE MOST USEFUL HOUSE PLANT 17 



will be placed in the kitchen. The fourth, also to be placed in 

 the kitchen, will consist of yeast put into the undiluted food, 

 and will be covered with rubber. 



In this set of experiments we have by no means represented 

 all the possible conditions, any more than, in the previous set, 

 we tried all the kinds of food. We will, however, be able to see 

 whether an ice-box temperature and darkness combined affect 

 this kind of fermentation, how darkness alone influences it, and the 

 result of a strong food solution. 



The next day look at the best three out of the five bottles. 

 You have here two proofs of the presence of a colorless gas. Can 

 we find out what it is? Here is a simple though inaccurate test, 

 safe in this case but not to be applied indiscriminately in other 

 experiments. 



Four colorless gases are hydroeen, oxygen, carbonic acid gas 

 (carbon dioxide), and nitrogen. Of course there are others, but 

 for this test these four wiU suffice. The first two support combus- 

 tion, will burn ; the others do not. Let us, therefore, light a splinter 

 of wood and hold it over the rubber, while we make an incision 

 to allow the gas to come out. This position is necessary because 

 some of the gases are lighter than air and rise rapidly. If the gas 

 is one of the first two mentioned there will be an explosion or at 

 least the splinter will burn more brightly. As nothing of the sort 

 happens we will enlarge the opening and plunge the splinter down 

 almost to the liquid. What is the result? 



We thus have proved in two ways that the gas is neither hydro- 

 gen nor oxygen. The next thing to do is to find out whether 

 carbonic acid gas or nitrogen remains. Of the two the former is 

 the easier to test and we can make a check experiment, for pur- 

 poses of comparison, by using the carbon dioxide given off from 

 our own lungs. 



Buy a little lime water at the drug store, as that is easier 

 than trying to make it. Pour part of it into a low dish. If you 

 breathe upon it you notice that a scum appears, and if you con- 

 tinue the whole liquid becomes milky. If you blow into it through 

 a glass tube the end of the tube touching the lime water is crusted 

 over. Now if you can find a cork to fit one of your bottles and 

 a glass tube bent like a blunt, inverted V, you can easily make the 

 gas do as you have done, namely, blow into lime water: that is, 

 if fermentation is still going on vigorously. It will thus tell you 



