May 24, 1901.] 
until it was clearly established that every 
distinct substance has a fixed and unalter- 
able composition. The second great law of 
combination was discovered in 1804 by 
John Dalton, and it iscommonly called the 
law of multiple proportions. To explain 
these laws of combination, Dalton intro- 
duced the atomic theory into chemistry, 
and from now on the great problem was to 
determine the relative weights of the atoms. 
When the history of chemistry in the nine- 
teenth century comes to be written, it will 
be largely the history of the atomic theory, 
and for more than sixty years the two great 
problems to which the most eminent men 
gave their attention were the determination 
of the atomic weights and of the arrange- 
ment of the atoms in compounds. It would 
be a long story to trace out step by step 
how these problems were solved. The men 
who did most in this direction were Berze- 
lius, Dumas, Liebig, Gerhardt and Laurent, 
Cannizzaro and Kekulé. As a result of 
their work, it began to be generally recog- 
nized, about 1865, that these two problems 
had been satisfactorily solved, and from that 
time on there has been no question as to 
the reasoning employed in fixing upon a 
number to represent the atomic weight of 
an element, or to determine the way in 
which the atoms are linked together in a 
compound. 
Side by side with this development of 
chemical theory has gone the discovery of 
new elements and compounds. Instead of 
the thirty elements or simple substances 
known at the beginning of the last century, 
we now have seventy-eight. Instead of a 
few scores of distinct compounds of definite 
composition,we now have thousands of these 
substances. To-day there are known 75,000 
compounds of carbon alone. 
1859 and 1860 Bunsen and Kirchoff devised 
the spectroscope, and it has become, next to 
the balance, the most important instrument 
of chemical investigation. By means of it 
SCIENCE 
In the years . 
805 
the elements rubidium, cesium, thallium, 
indium, gallium, scandium and helium have 
been discovered. 
THE PERIODIC LAW. 
Soon after the atomic weights had been 
determined satisfactorily, a very remarka- 
ble relationship was discovered by Lothar 
Meyer and Mendelejeff to exist between the 
atomic weights and the properties of the 
elements. It was found that when the ele- 
ments were arranged in the order of increas- 
ing atomic weights, beginning with the 
lowest and going up regularly to the highest, 
there was a periodic variation in the proper- 
ties of the elements. For example, it was 
noticed that the 8th element resembled the 
first, the 9th was analogous to the 2d, and 
so on. Mendelejeff expressed this fact in 
the following way: ‘‘ The properties of the 
elements,” he said, “ are a periodic function 
of their atomic weights.’’ By means of this 
law Mendelejeff was able to foretell the ex- 
istence of new elements and to predict their 
chemical and physical properties. When 
the table of elements was first arranged it 
was incomplete, there were blank spaces. 
Mendelejeff predicted that elements would 
be found that would fill these spaces, and 
from the properties of the adjoining ele- 
ments he foretold the properties of the un- 
known elements. In this way he predicted 
the properties of an element that would re- 
semble boron, another that would be anal- 
ogous to aluminum,and a third that would 
be closely related to silicon. ‘These predic- 
tions have all been fulfilled. In 1879 Nil- 
son discovered scandium and found that it 
had all of the properties of the unknown 
element that resembled boron, in 1875 Bois- 
baudran discovered gallium ; it was the ele: 
ment resembling aluminum, and in 1885 
Winkler discovered germanium; its proper- 
ties were almost identical with those that 
had been predicted for the element resem- 
bling silicon. 
