In 1868, he travelled with his father and Westerners invited by Mongkut to observe the solar eclipse of 18 August 1868 in Prachuap Khiri Khan Province.

All his reforms were dedicated to ensuring Siam's independence given the increasing encroachment of Western powers, so that Chulalongkorn earned the epithet Phra Piya Maharat or known as the Great Beloved King.

In 1867, King Mongkut led an expedition to the Malay Peninsula south of the city of Hua Hin,[3] to verify his calculations of the solar eclipse of 18 August 1868.

He supervised the digging of several important khlongs, such as Padung Krungkasem and Damneun Saduak, and the paving of roads such as Charoen Krung and Silom.

At the end of his regency, Sri Suriwongse was raised to Somdet Chao Phraya, the highest title a noble could attain.

[8] As tax collectors had been under the aegis of various nobles and thus a source of their wealth, this reform caused great consternation among the nobility, especially the Front Palace.

Prince Yodyingyot of the Front Palace was known to be on friendly terms with many Britons, at a time when Siamese relations with the British Empire were tense.

In the northern Laotian lands bordering China, the insurgents of the Taiping Rebellion had taken refuge since the reign of King Mongkut.

His Belgian advisor, Gustave Rolin-Jaequemyns, played a great role in the development of modern Siamese law and its judicial system.

The princes were influenced by the liberal notions of democracy and elections they encountered in republics like France and constitutional monarchies like the United Kingdom.

The most notable ones included Thianwan Wannapho, who had been imprisoned for 17 years and from prison produced many works criticizing traditional Siamese society.

In 1893 Auguste Pavie, the French vice-consul of Luang Prabang, requested the cession of all Laotian lands east of the Mekong River.

Inconstant and Comete were attacked in Chao Phraya, and the French sent an ultimatum: an indemnity of three million francs, as well as the cession of and withdrawal from Laos.

The issue was eventually settled with the cession of Laos in 1893, but the French troops in Chantaburi and Trat refused to leave.

An agreement was reached in 1904 that French troops would leave Chantaburi but hold the coast land from Trat to Koh Kong.

During a visit to Spain and Portugal, on 26 October, he condemned and ordered his servant to be executed for a breach of etiquette committed in Lisbon, according to the telegram news from Saragossa.

[12] Siam had been composed of a network of cities according to the Mandala system codified by King Trailokanat in 1454, with local rulers owing tribute to Bangkok.

During his European tour earlier that year, he had learned about the sanitary districts of England, and wanted to try out this local administrative unit in his capital.

With his experiences during the travel to British colonies and the suggestion of Prince Damrong, Chulalongkorn established the hierarchical system of monthons in 1897, composed of province, city, amphoe, tambon, and muban (village) in descending order.

[13] Ayutthaya King Ramathibodi II established a system of corvée in 1518 after which the lives of Siamese commoners and slaves were closely regulated by the government.

Chulalongkorn, to prevent such a bloodbath in Siam, provided several steps towards the abolition of slavery, not an extreme turning point from servitude to total freedom.

In 1874, Chulalongkorn enacted a law that lowered the redemption price of household slaves born in 1867 (his ascension year) and freed all of them when they had reached 21.

The reverse of 100 baht banknotes in circulation since the 2005 centennial depict Chulalongkorn in navy uniform abolishing the slave tradition.

The traditional corvée system declined after the Bowring Treaty, which gave rise to a new class of employed labourers not regulated by the government, while many noblemen continued to hold sway over large numbers of Phrai Som.

This was followed in 1907 by the first act providing for invoking martial law, which seven years later was changed to its modern form by his son and successor, King Vajiravudh.

Four sultanates (Kedah, Kelantan, Terengganu and Perlis) were brought under British influence in exchange for Siamese legal rights and a loan to construct railways in southern Siam.

In 1907, he visited his son's school in Britain and consulted with European doctors in pursuit of a cure for his kidney disease.

Beginning in Härnösand and travelling via Sollefteå and Ragunda he mounted a boat in the small village of Utanede in order to take him back through Sundsvall to Stockholm.

In 2005, the 100 baht banknote was revised to depict King Chulalongkorn in naval uniform and, in the background, abolishing slavery.

[23] The 1,000 baht banknote of Series 16, issued in 2015, depicts the King Chulalongkorn monument, Ananda Samakhom Throne Hall, and the abolition of slavery.