According to Dobereiner, all elements occurred in groups of three, when arranged in increasing order of atomic masses. He referred to these groups as triads. In a traid the elements had similar chemical properties. 

Traids of the Dobereiners classification:
Traid1:

Dobereiner’s law of triads states that, the atomic mass of the middle element of a triad is the arithmetic mean of the atomic masses of the other two elements.
Example: In the triad of lithium, sodium and potassium. The atomic mass of lithium is 7 and the atomic mass of potassium is 39. The average of masses of lithium and potassium gives atomic mass of sodium 23.

Main causes were:

Autocratic rule of Tsars: In 1914, the Russian emperor was Tsar Nicholas II. He fought a number of wars to expand his empire in the north and west in Europe. He had borne the expenditure of war by taxing the common people of Russia.
Conditions of peasants: Majority of the Russians were agriculturalists. Major part of the land was owned by nobles and clergy and these peasants worked as farmers on daily wages. They were paid less and worked more and sometimes under debt, they were not even paid wages.
Status of industries: Industry was found in pockets. Prominent industrial areas were St. Petersburg and Moscow. Craftsmen undertook much of the production, but large factories existed alongside crafts workshops. Foreign investment in industries increased with the extension of Russia’s railway network.
Conditions of workers in the industries: Most industries were owned by private
industrialists. Though the government supervised factories’ working hours and wages of the workers, but still rules were broken. Women workers were also paid less than men. Some workers formed associations to help members in times of unemployment and financial hardships. ,
Formation of socialist parties: All political parties were illegal in Russia before 1914. The Russian Social Democratic Workers Party was founded in 1898 by socialists, who respected Marxist ideas. But because of government policies, it had to operate secretly as an illegal organisation. It set up a newspaper, mobilised workers and organised strikes.

Main causes were:

Autocratic rule of Tsars: In 1914, the Russian emperor was Tsar Nicholas II. He fought a number of wars to expand his empire in the north and west in Europe. He had borne the expenditure of war by taxing the common people of Russia.
Conditions of peasants: Majority of the Russians were agriculturalists. Major part of the land was owned by nobles and clergy and these peasants worked as farmers on daily wages. They were paid less and worked more and sometimes under debt, they were not even paid wages.
Status of industries: Industry was found in pockets. Prominent industrial areas were St. Petersburg and Moscow. Craftsmen undertook much of the production, but large factories existed alongside crafts workshops. Foreign investment in industries increased with the extension of Russia’s railway network.
Conditions of workers in the industries: Most industries were owned by private
industrialists. Though the government supervised factories’ working hours and wages of the workers, but still rules were broken. Women workers were also paid less than men. Some workers formed associations to help members in times of unemployment and financial hardships. ,
Formation of socialist parties: All political parties were illegal in Russia before 1914. The Russian Social Democratic Workers Party was founded in 1898 by socialists, who respected Marxist ideas. But because of government policies, it had to operate secretly as an illegal organisation. It set up a newspaper, mobilised workers and organised strikes.

The period between the 1^st century B.C. to the end of 2^nd century A.D. in Southern India is known as Sangam Period. It has been named after the Sangam academies during that period.

According to the Tamil legends, there were three Sangams (Academy of Tamil poets) held in the ancient South India popularly called Muchchangam. These Sangams flourished under the royal patronage of the Pandya kings of Madurai.

1. The First Sangam, is believed to be held at Madurai, attended by gods and legendary sages. No literary work of this Sangam is available.
2. The Second Sangam was held at Kapadapuram, only Tolkappiyam survives from this.
3. The Third Sangam at Madurai was founded by Mudathirumaran. A few of these Tamil literary works have survived and are a useful sources to reconstruct the history of the Sangam period.