The nationalist leader who was arrested in 1897 on the charge of spreading hatred and disaffection against the government and sentenced to 18 months of rigorous imprisonment was Bal Gangadhar Tilak, commonly known as B.G. Tilak. He is considered one of the prominent leaders of the Indian independence movement and played a significant role in mobilizing the masses and creating a sense of national identity.

**Background of B.G. Tilak:**
- Bal Gangadhar Tilak was born on July 23, 1856, in Ratnagiri, Maharashtra, India.
- He was a scholar, journalist, and lawyer, known for his strong advocacy of Indian nationalism and his efforts to unite the country against British colonial rule.
- Tilak was one of the early leaders of the Indian National Congress and later became a prominent member of the extremist faction within the party.
- He strongly believed in the idea of Swaraj (self-rule) and worked tirelessly to awaken the Indian masses to the cause of independence.

**Arrest and Trial:**
- In 1897, Tilak was arrested by the British colonial government on charges of sedition for his writings and speeches that criticized the government and called for Indian self-rule.
- The British authorities accused Tilak of spreading hatred and disaffection against the government through his newspapers, Kesari (in Marathi) and Mahratta (in English).
- He was charged under Section 124A of the Indian Penal Code, which deals with sedition, and was sentenced to 18 months of rigorous imprisonment.

**Symbol of Self-Sacrifice:**
- During his imprisonment, Tilak became a symbol of self-sacrifice and resistance against British oppression.
- He used his time in prison to write and educate fellow prisoners about the freedom struggle and the importance of national unity.
- Tilak's imprisonment and his refusal to bow down to the British authorities garnered widespread support and admiration from the Indian masses.
- His popularity increased, and he emerged as a national hero and a rallying figure for the independence movement.

**Impact and Legacy:**
- Tilak's arrest and imprisonment further fueled the nationalist sentiments in India and intensified the demand for Swaraj.
- He inspired a new generation of leaders and activists who actively participated in the struggle for independence.
- Tilak's writings and speeches continue to inspire generations of Indians and his ideas on nationalism and self-rule remain relevant to this day.
- His contributions to the Indian independence movement earned him the title of "The Father of Indian Unrest."

In conclusion, Bal Gangadhar Tilak, through his arrest and subsequent imprisonment, became a living symbol of self-sacrifice and resistance against British colonial rule. His ideas and actions continue to inspire and influence the Indian nationalist movement, making him one of the most revered leaders in the country's history.