In this brigade, commanded by Colonel Burnside, they marched to the Battle of Bull Run, leading the column. On that sanguinary and disastrous field, it was the first, with Captain Reynolds battery, to engage, and fought the enemy forty-five minutes without support, losing 28 men killed, 56 wounded, and 30 missing; among the former, Colonel Slocum, Major Sullivan Ballou, and Captains Levi A. Tower and Samuel J. Smith. The men stood up bravely under a heavy fire from the rebel batteries, but to no purpose. The color company was a conspicuous mark, and the regimental colors were completely riddled by balls. Dr. James Harris, Surgeon of the Regiment, was unceasing in the performance of his professional duties through the day, often exposed to danger on the field, and always having words of cheer for the wounded and dying. After the retreat commenced, he remained at his post, and gave himself up a prisoner, rather than be separated from those who so much needed his attention. The death of the brave Colonel Slocum, left the Regiment in the command of Captain Frank Wheaton, of the United States Army, then acting Lieutenant-Colonel, to the Colonelcy of which he was subsequently promoted. Captain Viall, on the fall of Major Ballou, assumed the duty of a field officer, and was afterward promoted to Major of the Regiment. Captain William H. P. Steere received the commission of Lieutenant-Colonel in the same. In retiring from the field, the Regiment preserved its order, and on returning to Washington established temporary quarters at Camp Clark.